Summa Th. II-II EN Qu.96 a.4

IRRELIGION, i.e. BY WAY OF DEFICIENCY (Questions [97]-102)


We must now consider the vices that are opposed to religion, through lack of religion, and which are manifestly contrary thereto, so that they come under the head of irreligion. Such are the vices which pertain to contempt or irreverence for God and holy things. Accordingly we shall consider: (1) Vices pertaining directly to irreverence for God; (2) Vices pertaining to irreverence for holy things. With regard to the first we shall consider the temptation whereby God is tempted, and perjury, whereby God's name is taken with irreverence. Under the first head there are four points of inquiry:

(1) In what the temptation of God consists;

(2) Whether it is a sin?

(3) To what virtue it is opposed;

(4) Of its comparison with other vices.

Whether the temptation of God consists in certain deeds, wherein the expected result is ascribed to the power of God alone?

Objection: 1. It would seem that the temptation of God does not consist in certain deeds wherein the result is expected from the power of God alone. Just as God is tempted by man so is man tempted by God, man, and demons. But when man is tempted the result is not always expected from his power. Therefore neither is God tempted when the result is expected from His power alone.
2. Further, all those who work miracles by invoking the divine name look for an effect due to God's power alone. Therefore, if the temptation of God consisted in such like deeds, all who work miracles would tempt God.
3. Further, it seems to belong to man's perfection that he should put aside human aids and put his hope in God alone. Hence Ambrose, commenting on Lc 9,3, "Take nothing for your journey," etc. says: "The Gospel precept points out what is required of him that announces the kingdom of God, namely, that he should not depend on worldly assistance, and that, taking assurance from his faith, he should hold himself to be the more able to provide for himself, the less he seeks these things." And the Blessed Agatha said: "I have never treated my body with bodily medicine, I have my Lord Jesus Christ, Who restores all things by His mere word." [*Office of St. Agatha, eighth Responsory (Dominican Breviary).] But the temptation of God does not consist in anything pertaining to perfection. Therefore the temptation of God does not consist in such like deeds, wherein the help of God alone is expected.

On the contrary Augustine says (Contra Faust. xxii, 36): "Christ who gave proof of God's power by teaching and reproving openly, yet not allowing the rage of His enemies to prevail against Him, nevertheless by fleeing and hiding, instructed human weakness, lest it should dare to tempt God when it has to strive to escape from that which it needs to avoid." From this it would seem that the temptation of God consists in omitting to do what one can in order to escape from danger, and relying on the assistance of God alone.
I answer that Properly speaking, to tempt is to test the person tempted. Now we put a person to the test by words or by deeds. By words, that we may find out whether he knows what we ask, or whether he can and will grant it: by deeds, when, by what we do, we probe another's prudence, will or power. Either of these may happen in two ways. First, openly, as when one declares oneself a tempter: thus Samson (Jg 14,12) proposed a riddle to the Philistines in order to tempt them. In the second place it may be done with cunning and by stealth, as the Pharisees tempted Christ, as we read in Mt 22,15, sqq. Again this is sometimes done explicitly, as when anyone intends, by word or deed, to put some person to the test; and sometimes implicitly, when, to wit, though he does not intend to test a person, yet that which he does or says can seemingly have no other purpose than putting him to a test.Accordingly, man tempts God sometimes by words, sometimes by deeds. Now we speak with God in words when we pray. Hence a man tempts God explicitly in his prayers when he asks something of God with the intention of probing God's knowledge, power or will. He tempts God explicitly by deeds when he intends, by whatever he does, to experiment on God's power, good will or wisdom. But He will tempt God implicitly, if, though he does not intend to make an experiment on God, yet he asks for or does something which has no other use than to prove God's power, goodness or knowledge. Thus when a man wishes his horse to gallop in order to escape from the enemy, this is not giving the horse a trial: but if he make the horse gallop with out any useful purpose, it seems to be nothing else than a trial of the horse's speed; and the same applies to all other things. Accordingly when a man in his prayers or deeds entrusts himself to the divine assistance for some urgent or useful motive, this is not to tempt God: for it is written (2Ch 20,12): "As we know not what to do, we can only turn our eyes to Thee." But if this be done without any useful or urgent motive, this is to tempt God implicitly. Wherefore a gloss on Dt 6,16, "Thou shalt not tempt the Lord thy God," says: "A man tempts God, if having the means at hand, without reason he chooses a dangerous course, trying whether he can be delivered by God."

Reply to Objection: 1. Man also is sometimes tempted by means of deeds, to test his ability or knowledge or will to uphold or oppose those same deeds.
2. When saints work miracles by their prayers, they are moved by a motive of necessity or usefulness to ask for that which is an effect of the divine power.
3. The preachers of God's kingdom dispense with temporal aids, so as to be freer to give their time to the word of God: wherefore if they depend on God alone, it does not follow that they tempt God. But if they were to neglect human assistance without any useful or urgent motive, they would be tempting God. Hence Augustine (Contra Faust. xxii, 36) says that "Paul fled, not through ceasing to believe in God, but lest he should tempt God, were he not to flee when he had the means of flight." The Blessed Agatha had experience of God's kindness towards her, so that either she did not suffer such sickness as required bodily medicine, or else she felt herself suddenly cured by God.

Whether it is a sin to tempt God?

Objection: 1. It would seem that it is not a sin to tempt God. For God has not commanded sin. Yet He has commanded men to try, which is the same as to tempt, Him: for it is written (Ml 3,10): "Bring all the tithes into the storehouse, that there may be meat in My house; and try Me in this, saith the Lord, if I open not unto you the flood-gates of heaven." Therefore it seems not to be a sin to tempt God.
2. Further, a man is tempted not only in order to test his knowledge and his power, but also to try his goodness or his will. Now it is lawful to test the divine goodness or will, for it is written (Ps 33,9): "O taste and see that the Lord is sweet," and (Rm 12,2): "That you may prove what is the good, and the acceptable, and the perfect will of God." Therefore it is not a sin to tempt God.
3. Further, Scripture never blames a man for ceasing from sin, but rather for committing a sin. Now Achaz is blamed because when the Lord said: "Ask thee a sign of the Lord thy God," he replied: "I will not ask, and I will not tempt the Lord," and then it was said to him: "Is it a small thing for you to be grievous to men, that you are grievous to my God also?" (Is 7,11-13). And we read of Abraham (Gn 15,8) that he said to the Lord: "Whereby may I know that I shall possess it?" namely, the land which God had promised him. Again Gedeon asked God for a sign of the victory promised to him (Jg 6,36, sqq.). Yet they were not blamed for so doing. Therefore it is not a sin to tempt God.

On the contrary It is forbidden in God's Law, for it is written (Dt 6,10): "Thou shalt not tempt the Lord thy God."
I answer that As stated above (Article [1]), to tempt a person is to put him to a test. Now one never tests that of which one is certain. Wherefore all temptation proceeds from some ignorance or doubt, either in the tempter (as when one tests a thing in order to know its qualities), or in others (as when one tests a thing in order to prove it to others), and in this latter way God is said to tempt us. Now it is a sin to be ignorant of or to doubt that which pertains to God's perfection. Wherefore it is evident that it is a sin to tempt God in order that the tempter himself may know God's power.On the other hand, if one were to test that which pertains to the divine perfection, not in order to know it oneself, but to prove it to others: this is not tempting God, provided there be just motive of urgency, or a pious motive of usefulness, and other requisite conditions. For thus did the apostles ask the Lord that signs might be wrought in the name of Jesus Christ, as related in Ac 4,30, in order, to wit, that Christ's power might be made manifest to unbelievers.

Reply to Objection: 1. The paying of tithes was prescribed in the Law, as stated above (Question [87], Article [1]). Hence there was a motive of urgency to pay it, through the obligation of the Law, and also a motive of usefulness, as stated in the text quoted---"that there may be meat in God's house": wherefore they did not tempt God by paying tithes. The words that follow, "and try Me," are not to be understood causally, as though they had to pay tithes in order to try if "God would open the flood-gates of heaven," but consecutively, because, to wit, if they paid tithes, they would prove by experience the favors which God would shower upon them.
2. There is a twofold knowledge of God's goodness or will. One is speculative and as to this it is not lawful to doubt or to prove whether God's will be good, or whether God is sweet. The other knowledge of God's will or goodness is effective or experimental and thereby a man experiences in himself the taste of God's sweetness, and complacency in God's will, as Dionysius says of Hierotheos (Div. Nom. ii) that "he learnt divine thing through experience of them." It is in this way that we are told to prove God's will, and to taste His sweetness.
3. God wished to give a sign to Achaz, not for him alone, but for the instruction of the whole people. Hence he was reproved because, by refusing to ask a sign, he was an obstacle to the common welfare. Nor would he have tempted God by asking, both because he would have asked through God commanding him to do so, and because it was a matter relating to the common good. Abraham asked for a sign through the divine instinct, and so he did not sin. Gedeon seems to have asked a sign through weakness of faith, wherefore he is not to be excused from sin, as a gloss observes: just as Zachary sinned in saying to the angel (Lc 1,18): "Whereby shall I know this?" so that he was punished for his unbelief.It must be observed, however, that there are two ways of asking God for a sign: first in order to test God's power or the truth of His word, and this of its very nature pertains to the temptation of God. Secondly, in order to be instructed as to what is God's pleasure in some particular matter; and this nowise comes under the head of temptation of God.

Whether temptation of God is opposed to the virtue of religion?

Objection: 1. It would seem that the temptation of God is not opposed to the virtue of religion. The temptation of God is sinful, because a man doubts God, as stated above (Article [2]). Now doubt about God comes under the head of unbelief, which is opposed to faith. Therefore temptation of God is opposed to faith rather than to religion.
2. Further, it is written (Si 18,23): "Before prayer prepare thy soul, and be not as a man that tempteth God. Such a man," that is, who tempts God, says the interlinear gloss, "prays for what God taught him to pray for, yet does not what God has commanded him to do." Now this pertains to imprudence which is opposed to hope. Therefore it seems that temptation of God is a sin opposed to hope.
3. Further, a gloss on Ps 77,18, "And they tempted God in their hearts," says that "to tempt God is to pray to Him deceitfully, with simplicity in our words and wickedness in our hearts." Now deceit is opposed to the virtue of truth. Therefore temptation of God is opposed, not to religion, but to truth.

On the contrary According to the gloss quoted above "to tempt God is to pray to Him inordinately." Now to pray to God becomingly is an act of religion as stated above (Question [83], Article [15]). Therefore to tempt God is a sin opposed to religion.
I answer that As clearly shown above (Question [81], Article [5]), the end of religion is to pay reverence to God. Wherefore whatever pertains directly to irreverence for God is opposed to religion. Now it is evident that to tempt a person pertains to irreverence for him: since no one presumes to tempt one of whose excellence he is sure. Hence it is manifest that to tempt God is a sin opposed to religion.

Reply to Objection: 1. As stated above (Question [81], Article [7]), it belongs to religion to declare one's faith by certain signs indicative of reverence towards God. Consequently it belongs to irreligion that, through doubtful faith, a man does things indicative of irreverence towards God. To tempt God is one of these; wherefore it is a species of irreligion.
2. He that prepares not his soul before prayer by forgiving those against whom he has anything, or in some other way disposing himself to devotion, does not do what he can to be heard by God, wherefore he tempts God implicitly as it were. And though this implicit temptation would seem to arise from presumption or indiscretion, yet the very fact that a man behaves presumptuously and without due care in matters relating to God implies irreverence towards Him. For it is written (1P 5,6): "Be you humbled . . . under the mighty hand of God," and (2Tm 2,15): "Carefully study to present thyself approved unto God." Therefore also this kind of temptation is a species of irreligion.
3. A man is said to pray deceitfully, not in relation to God, Who knows the secrets of the heart, but in relation to man. Wherefore deceit is accidental to the temptation of God, and consequently it does not follow that to tempt God is directly opposed to the truth.

Whether the temptation of God is a graver sin than superstition?

Objection: 1. It would seem that the temptation of God is a graver sin than superstition. The greater sin receives the greater punishment. Now the sin of tempting God was more severely punished in the Jews than was the sin of idolatry; and yet the latter is the chief form of superstition: since for the sin of idolatry three thousand men of their number were slain, as related in Ex 32,28 [*Septuagint version. The Vulgate has "twenty-three thousand."], whereas for the sin of temptation they all without exception perished in the desert, and entered not into the land of promise, according to Ps 94,9, "Your fathers tempted Me," and further on, "so I swore in My wrath that they should not enter into My rest." Therefore to tempt God is a graver sin than superstition.
2. Further, the more a sin is opposed to virtue the graver it would seem to be. Now irreligion, of which the temptation of God is a species, is more opposed to the virtue of religion, than superstition which bears some likeness to religion. Therefore to tempt God is a graver sin than superstition.
3. Further, it seems to be a greater sin to behave disrespectfully to one's parents, than to pay others the respect we owe to our parents. Now God should be honored by us as the Father of all (Ml 1,6). Therefore. temptation of God whereby we behave irreverently to God, seems to be a greater sin than idolatry, whereby we give to a creature the honor we owe to God.

On the contrary A gloss on Dt 17,2, "When there shall be found among you," etc. says: "The Law detests error and idolatry above all: for it is a very great sin to give to a creature the honor that belongs to the Creator."
I answer that Among sins opposed to religion, the more grievous is that which is the more opposed to the reverence due to God. Now it is less opposed to this reverence that one should doubt the divine excellence than that one should hold the contrary for certain. For just as a man is more of an unbeliever if he be confirmed in his error, than if he doubt the truth of faith, so, too, a man acts more against the reverence due to God, if by his deeds he professes an error contrary to the divine excellence, than if he expresses a doubt. Now the superstitious man professes an error, as shown above (Question [94], Article [1], ad 1), whereas he who tempts God by words or deeds expresses a doubt of the divine excellence, as stated above (Article [2]). Therefore the sin of superstition is graver than the sin of tempting God.

Reply to Objection: 1. The sin of idolatry was not punished in the above manner, as though it were a sufficient punishment; because a more severe punishment was reserved in the future for that sin, for it is written (Ex 32,34): "And I, in the day of revenge, will visit this sin also of theirs."
2. Superstition bears a likeness to religion, as regards the material act which it pays just as religion does. But, as regards the end, it is more contrary to religion than the temptation of God, since it implies greater irreverence for God, as stated.
3. It belongs essentially to the divine excellence that it is singular and incommunicable. Consequently to give divine reverence to another is the same as to do a thing opposed to the divine excellence. There is no comparison with the honor due to our parents, which can without sin be given to others.


We must now consider perjury: under which head there are four points of inquiry:

(1) Whether falsehood is necessary for perjury?

(2) Whether perjury is always a sin?

(3) Whether it is always a mortal sin?

(4) Whether it is a sin to enjoin an oath on a perjurer?

Whether it is necessary for perjury that the statement confirmed on oath be false?

Objection: 1. It would seem that it is not necessary for perjury that the statement confirmed on oath be false. As stated above (Question [89], Article [3]), an oath should be accompanied by judgment and justice no less than by truth. Since therefore perjury is incurred through lack of truth, it is incurred likewise through lack of judgment, as when one swears indiscreetly, and through lack of justice, as when one swears to something unjust.
2. Further, that which confirms is more weighty than the thing confirmed thereby: thus in a syllogism the premises are more weighty than the conclusion. Now in an oath a man's statement is confirmed by calling on the name of God. Therefore perjury seems to consist in swearing by false gods rather than in a lack of truth in the human statement which is confirmed on oath.
3. Further, Augustine says (De Verb. Apost. Jacobi; Serm. clxxx): "Men swear falsely both in deceiving others and when they are deceived themselves"; and he gives three examples. The first is: "Supposing a man to swear, thinking that what he swears to is true, whereas it is false"; the second is: "Take the instance of another who knows the statement to be false, and swears to it as though it were true"; and the third is: "Take another, who thinks his statement false, and swears to its being true, while perhaps it is true," of whom he says afterwards that he is a perjurer. Therefore one may be a perjurer while swearing to the truth. Therefore falsehood is not necessary for perjury.

On the contrary Perjury is defined "a falsehood confirmed by oath" [*Hugh of St. Victor, Sum. Sent. iv, 5].
I answer that As stated above (Question [92], Article [2]), moral acts take their species from their end. Now the end of an oath is the confirmation of a human assertion. To this confirmation falsehood is opposed: since an assertion is confirmed by being firmly shown to be true; and this cannot happen to that which is false. Hence falsehood directly annuls the end of an oath: and for this reason, that perversity in swearing, which is called perjury, takes its species chiefly from falsehood. Consequently falsehood is essential to perjury.

Reply to Objection: 1. As Jerome says on Jr 4,2, "whichever of these three be lacking, there is perjury," but in different order. For first and chiefly perjury consists in a lack of truth, for the reason stated in the Article. Secondly, there is perjury when justice is lacking, for in whatever way a man swears to that which is unlawful, for this very reason he is guilty of falsehood, since he is under an obligation to do the contrary. Thirdly, there is perjury when judgment is lacking, since by the very fact that a man swears indiscreetly, he incurs the danger of lapsing into falsehood.
2. In syllogisms the premises are of greater weight, since they are in the position of active principle, as stated in Phys. ii, 3: whereas in moral matters the end is of greater importance than the active principle. Hence though it is a perverse oath when a man swears to the truth by false gods, yet perjury takes its name from that kind of perversity in an oath, that deprives the oath of its end, by swearing what is false.
3. Moral acts proceed from the will, whose object is the apprehended good. Wherefore if the false be apprehended as true, it will be materially false, but formally true, as related to the will. If something false be apprehended as false, it will be false both materially and formally. If that which is true be apprehended as false, it will be materially true, and formally false. Hence in each of these cases the conditions required for perjury are to be found in some way, on account of some measure of falsehood. Since, however, that which is formal in anything is of greater importance than that which is material, he that swears to a falsehood thinking it true is not so much of a perjurer as he that swears to the truth thinking it false. For Augustine says (De Verb. Apost. Jacobi; Serm. clxxx): "It depends how the assertion proceeds from the mind, for the tongue is not guilty except the mind be guilty."

Whether all perjury is sinful?

Objection: 1. It would seem that not all perjury is sinful. Whoever does not fulfil what he has confirmed on oath is seemingly a perjurer. Yet sometimes a man swears he will do something unlawful (adultery, for instance, or murder): and if he does it, he commits a sin. If therefore he would commit a sin even if he did it not, it would follow that he is perplexed.
2. Further, no man sins by doing what is best. Yet sometimes by committing a perjury one does what is best: as when a man swears not to enter religion, or not to do some kind of virtuous deed. Therefore not all perjury is sinful.
3. Further, he that swears to do another's will would seem to be guilty of perjury unless he do it. Yet it may happen sometimes that he sins not, if he do not the man's will: for instance, if the latter order him to do something too hard and unbearable. Therefore seemingly not all perjury is sinful.
4. Further, a promissory oath extends to future, just as a declaratory oath extends to past and present things. Now the obligation of an oath may be removed by some future occurrence: thus a state may swear to fulfil some obligation, and afterwards other citizens come on the scene who did not take the oath; or a canon may swear to keep the statutes of a certain church, and afterwards new statutes are made. Therefore seemingly he that breaks an oath does not sin.

On the contrary Augustine says (De Verb. Apost. Jacobi; Serm. cxxx), in speaking of perjury: "See how you should detest this horrible beast and exterminate it from all human business."
I answer that As stated above (Question [89], Article [1]), to swear is to call God as witness. Now it is an irreverence to God to call Him to witness to a falsehood, because by so doing one implies either that God ignores the truth or that He is willing to bear witness to a falsehood. Therefore perjury is manifestly a sin opposed to religion, to which it belongs to show reverence to God.

Reply to Objection: 1. He that swears to do what is unlawful is thereby guilty of perjury through lack of justice: though, if he fails to keep his oath, he is not guilty of perjury in this respect, since that which he swore to do was not a fit matter of an oath.
2. A person who swears not to enter religion, or not to give an alms, or the like, is guilty of perjury through lack of judgment. Hence when he does that which is best it is not an act of perjury, but contrary thereto: for the contrary of that which he is doing could not be a matter of an oath.
3. When one man swears or promises to do another's will, there is to be understood this requisite condition---that the thing commanded be lawful and virtuous, and not unbearable or immoderate.
4. An oath is a personal act, and so when a man becomes a citizen of a state, he is not bound, as by oath, to fulfil whatever the state has sworn to do. Yet he is bound by a kind of fidelity, the nature of which obligation is that he should take his share of the state's burdens if he takes a share of its goods.The canon who swears to keep the statutes that have force in some particular "college" is not bound by his oath to keep any that may be made in the future, unless he intends to bind himself to keep all, past and future. Nevertheless he is bound to keep them by virtue of the statutes themselves, since they are possessed of coercive force, as stated above (FS, Question [96], Article [4]).

Whether all perjury is a mortal sin?

Objection: 1. It would seem that not all perjury is a mortal sin. It is laid down (Extra, De Jurejur, cap. Verum): "Referring to the question whether an oath is binding on those who have taken one in order to safeguard their life and possessions, we have no other mind than that which our predecessors the Roman Pontiffs are known to have had, and who absolved such persons from the obligations of their oath. Henceforth, that discretion may be observed, and in order to avoid occasions of perjury, let them not be told expressly not to keep their oath: but if they should not keep it, they are not for this reason to be punished as for a mortal sin." Therefore not all perjury is a mortal sin.
2. Further, as Chrysostom [*Hom. xliv in the Opus Imperfectum on St. Matthew, falsely ascribed to St. John Chrysostom] says, "it is a greater thing to swear by God than by the Gospels." Now it is not always a mortal sin to swear by God to something false; for instance, if we were to employ such an oath in fun or by a slip of the tongue in the course of an ordinary conversation. Therefore neither is it always a mortal sin to break an oath that has been taken solemnly on the Gospels.
3. Further, according to the Law a man incurs infamy through committing perjury (VI, qu. i, cap. Infames). Now it would seem that infamy is not incurred through any kind of perjury, as it is prescribed in the case of a declaratory oath violated by perjury [*Cap. Cum dilectus, de Ord. Cognit.]. Therefore, seemingly, not all perjury is a mortal sin.

On the contrary Every sin that is contrary to a divine precept is a mortal sin. Now perjury is contrary to a divine precept, for it is written (Lv 19,12): "Thou shalt not swear falsely by My name." Therefore it is a mortal sin.
I answer that According to the teaching of the Philosopher (Poster. i, 2), "that which causes a thing to be such is yet more so." Now we know that an action which is, by reason of its very nature, a venial sin, or even a good action, is a mortal sin if it be done out of contempt of God. Wherefore any action that of its nature, implies contempt of God is a mortal sin. Now perjury, of its very nature implies contempt of God, since, as stated above (Article [2]), the reason why it is sinful is because it is an act of irreverence towards God. Therefore it is manifest that perjury, of its very nature, is a mortal sin.

Reply to Objection: 1. As stated above (Question [89], Article [7], ad 3), coercion does not deprive a promissory oath of its binding force, as regards that which can be done lawfully. Wherefore he who fails to fulfil an oath which he took under coercion is guilty of perjury and sins mortally. Nevertheless the Sovereign Pontiff can, by his authority, absolve a man from an obligation even of an oath, especially if the latter should have been coerced into taking the oath through such fear as may overcome a high-principled man.When, however, it is said that these persons are not to be punished as for a mortal sin, this does not mean that they are not guilty of mortal sin, but that a lesser punishment is to be inflicted on them.
2. He that swears falsely in fun is nonetheless irreverent to God, indeed, in a way, he is more so, and consequently is not excused from mortal sin. He that swears falsely by a slip of tongue, if he adverts to the fact that he is swearing, and that he is swearing to something false, is not excused from mortal sin, as neither is he excused from contempt of God. If, however, he does not advert to this, he would seem to have no intention of swearing, and consequently is excused from the sin of perjury.It is, however, a more grievous sin to swear solemnly by the Gospels, than to swear by God in ordinary conversation, both on account of scandal and on account of the greater deliberation. But if we consider them equally in comparison with one another, it is more grievous to commit perjury in swearing by God than in swearing by the Gospels.
3. Not every sin makes a man infamous in the eye of the law. Wherefore, if a man who has sworn falsely in a declaratory oath be not infamous in the eye of the law, but only when he has been so declared by sentence in a court of law, it does not follow that he has not sinned mortally. The reason why the law attaches infamy rather to one who breaks a promissory oath taken solemnly is that he still has it in his power after he has sworn to substantiate his oath, which is not the case in a declaratory oath.

Whether he sins who demands an oath of a perjurer?

Objection: 1. It would seem that he who demands an oath of a perjurer commits a sin. Either he knows that he swears truly, or he knows that he swears falsely. If he knows him to swear truly, it is useless for him to demand an oath: and if he believes him to swear falsely, for his own part he leads him into sin. Therefore nowise seemingly should one enjoin an oath on another person.
2. Further, to receive an oath from a person is less than to impose an oath on him. Now it would seem unlawful to receive an oath from a person, especially if he swear falsely, because he would then seem to consent in his sin. Much less therefore would it seem lawful to impose an oath on one who swears falsely.
3. Further, it is written (Lv 5,1): "If anyone sin, and hear the voice of one swearing falsely [*'Falsely' is not in the Vulgate'], and is a witness either because he himself hath seen, or is privy to it: if he do not utter it, he shall bear his iniquity." Hence it would seem that when a man knows another to be swearing falsely, he is bound to denounce him. Therefore it is not lawful to demand an oath of such a man.

On the contrary On the other hand, Just as it is a sin to swear falsely so is it to swear by false gods. Yet it is lawful to take advantage of an oath of one who has sworn by false gods, as Augustine says (ad Public. Ep. xlvii). Therefore it is lawful to demand an oath from one who swears falsely.
I answer that As regards a person who demands an oath from another, a distinction would seem to be necessary. For either he demands the oath on his own account and of his own accord, or he demands it on account of the exigencies of a duty imposed on him. If a man demands an oath on his own account as a private individual, we must make a distinction, as does Augustine (de Perjuriis. serm. clxxx): "For if he knows not that the man will swear falsely, and says to him accordingly: 'Swear to me' in order that he may be credited, there is no sin: yet it is a human temptation" (because, to wit, it proceeds from his weakness in doubting whether the man will speak the truth). "This is the evil whereof Our Lord says (Mt 5,37): That which is over and above these, is of evil. But if he knows the man to have done so," i.e. the contrary of what he swears to, "and yet forces him to swear, he is a murderer: for the other destroys himself by his perjury, but it is he who urged the hand of the slayer."If, on the other hand, a man demands an oath as a public person, in accordance with the requirements of the law, on the requisition of a third person: he does not seem to be at fault, if he demands an oath of a person, whether he knows that he will swear falsely or truly, because seemingly it is not he that exacts the oath but the person at whose instance he demands it.

Reply to Objection: 1. This argument avails in the case of one who demands an oath on his own account. Yet he does not always know that the other will swear truly or falsely, for at times he has doubts about the fact, and believes he will swear truly. In such a case he exacts an oath in order that he may be more certain.
2. As Augustine says (ad Public. serm. xlvii), "though we are forbidden to swear, I do not remember ever to have read in the Holy Scriptures that we must not accept oaths from others." Hence he that accepts an oath does not sin, except perchance when of his own accord he forces another to swear, knowing that he will swear falsely.
3. As Augustine says (Questions. Super Lev, qu. i), Moses in the passage quoted did not state to whom one man had to denounce another's perjury: wherefore it must be understood that the matter had to be denounced "to those who would do the perjurer good rather than harm." Again, neither did he state in what order the denunciation was to be made: wherefore seemingly the Gospel order should be followed, if the sin of perjury should be hidden, especially when it does not tend to another person's injury: because if it did, the Gospel order would not apply to the case, as stated above (Question [33], Article [7]; Question [68], Article [1]).
4. It is lawful to make use of an evil for the sake of good, as God does, but it is not lawful to lead anyone to do evil. Consequently it is lawful to accept the oath of one who is ready to swear by false gods, but it is not lawful to induce him to swear by false gods. Yet it seems to be different in the case of one who swears falsely by the true God, because an oath of this kind lacks the good of faith, which a man makes use of in the oath of one who swears truly by false gods, as Augustine says (ad Public. Ep. xlvii). Hence when a man swears falsely by the true God his oath seems to lack any good that one may use lawfully.

Summa Th. II-II EN Qu.96 a.4