Summa Th. II-II EN Qu.87 a.2

Whether men are bound to pay tithes of all things?

Objection: 1. It would seem that men are not bound to give tithes of all things. The paying of tithes seems to be an institution of the Old Law. Now the Old Law contains no precept about personal tithes, viz. those that are payable on property acquired by one's own act, for instance by commerce or soldiering. Therefore no man is bound to pay tithes on such things.
2. Further, it is not right to make oblations of that which is ill-gotten, as stated above (Question [86], Article [3]). Now oblations, being offered to God immediately, seem to be more closely connected with the divine worship than tithes which are offered to the ministers. Therefore neither should tithes be paid on ill-gotten goods.
3. Further, in the last chapter of Leviticus (Lv 30,32) the precept of paying tithes refers only to "corn, fruits of trees" and animals "that pass under the shepherd's rod." But man derives a revenue from other smaller things, such as the herbs that grow in his garden and so forth. Therefore neither on these things is a man bound to pay tithes.
4. Further, man cannot pay except what is in his power. Now a man does not always remain in possession of all his profit from land and stock, since sometimes he loses them by theft or robbery; sometimes they are transferred to another person by sale; sometimes they are due to some other person, thus taxes are due to princes, and wages due to workmen. Therefore one ought not to pay tithes on such like things.

On the contrary It is written (Gn 28,22): "Of all things that Thou shalt give to me, I will offer tithes to Thee."
I answer that In judging about a thing we should look to its principle. Now the principle of the payment of tithes is the debt whereby carnal things are due to those who sow spiritual things, according to the saying of the Apostle (1Co 9,11), "If we have sown unto you spiritual things, is it a great matter if we reap your carnal things?" [thus implying that on the contrary "it is no great matter if we reap your carnal things"] [*The phrase in the brackets is omitted in the Leonine edition]. For this debt is the principle on which is based the commandment of the Church about the payment of tithes. Now whatever man possesses comes under the designation of carnal things. Therefore tithes must be paid on whatever one possesses.

Reply to Objection: 1. In accordance with the condition of that people there was a special reason why the Old Law did not include a precept about personal tithes; because, to wit, all the other tribes had certain possessions wherewith they were able to provide a sufficient livelihood for the Levites who had no possessions, but were not forbidden to make a profit out of other lawful occupations as the other Jews did. On the other hand the people of the New Law are spread abroad throughout the world, and many of them have no possessions, but live by trade, and these would contribute nothing to the support of God's ministers if they did not pay tithes on their trade profits. Moreover the ministers of the New Law are more strictly forbidden to occupy themselves in money-making trades, according to 2Tm 2,4, "No man being a soldier to God, entangleth himself with secular business." Wherefore in the New Law men are bound to pay personal tithes, according to the custom of their country and the needs of the ministers: hence Augustine, whose words are quoted 16, qu. 1, cap. Decimae, says [*Append. Serm. cclxxvii]: "Tithes must be paid on the profits of soldiering, trade or craft."
2. Things are ill-gotten in two ways. First, because the getting itself was unjust: such, for instance, are things gotten by robbery, theft or usury: and these a man is bound to restore, and not to pay tithes on them. If, however, a field be bought with the profits of usury, the usurer is bound to pay tithes on the produce, because the latter is not gotten usuriously but given by God. On the other hand certain things are said to be ill-gotten, because they are gotten of a shameful cause, for instance of whoredom or stage-playing, and the like. Such things a man is not bound to restore, and consequently he is bound to pay tithes on them in the same way as other personal tithes. Nevertheless the Church must not accept the tithe so long as those persons remain in sin, lest she appear to have a share in their sins: but when they have done penance, tithes may be accepted from them on these things.
3. Things directed to an end must be judged according to their fittingness to the end. Now the payment of tithes is due not for its own sake, but for the sake of the ministers, to whose dignity it is unbecoming that they should demand minute things with careful exactitude, for this is reckoned sinful according to the Philosopher (Ethic. iv, 2). Hence the Old Law did not order the payment of tithes on such like minute things, but left it to the judgment of those who are willing to pay, because minute things are counted as nothing. Wherefore the Pharisees who claimed for themselves the perfect justice of the Law, paid tithes even on these minute things: nor are they reproved by our Lord on that account, but only because they despised greater, i.e. spiritual, precepts; and rather did He show them to be deserving of praise in this particular, when He said (Mt 23,23): "These things you ought to have done," i.e. during the time of the Law, according to Chrysostom's [*Hom. xliv in the Opus Imperfectum falsely ascribed to St. John Chrysostom] commentary. This also seems to denote fittingness rather than obligation. Therefore now too men are not bound to pay tithes on such minute things, except perhaps by reason of the custom of one's country.
4. A man is not bound to pay tithes on what he has lost by theft or robbery, before he recovers his property: unless he has incurred the loss through his own fault or neglect, because the Church ought not to be the loser on that account. If he sell wheat that has not been tithed, the Church can command the tithes due to her, both from the buyer who has a thing due to the Church, and from the seller, because so far as he is concerned he has defrauded the Church: yet if one pays, the other is not bound. Tithes are due on the fruits of the earth, in so far as these fruits are the gift of God. Wherefore tithes do not come under a tax, nor are they subject to workmen's wages. Hence it is not right to deduct one's taxes and the wages paid to workmen, before paying tithes: but tithes must be paid before anything else on one's entire produce.

Whether tithes should be paid to the clergy?

Objection: 1. It would seem that tithes should not be paid to the clergy. Tithes were paid to the Levites in the Old Testament, because they had no portion in the people's possessions, according to Nb 18,23-24. But in the New Testament the clergy have possessions not only ecclesiastical, but sometimes also patrimonial: moreover they receive first-fruits, and oblations for the living and the dead. Therefore it is unnecessary to pay tithes to them.
2. Further, it sometimes happens that a man dwells in one parish, and farms in another; or a shepherd may take his flock within the bounds of one parish during one part of the year, and within the bounds of one parish during one part of the year, and within the bounds of another parish during the other part of the year; or he may have his sheepfold in one parish, and graze the sheep in another. Now in all these and similar cases it seems impossible to decide to which clergy the tithes ought to be paid. Therefore it would seem that no fixed tithe ought to be paid to the clergy.
3. Further, it is the general custom in certain countries for the soldiers to hold the tithes from the Church in fee; and certain religious receive tithes. Therefore seemingly tithes are not due only to those of the clergy who have care of souls.

On the contrary It is written (Nb 18,21): "I have given to the sons of Levi all the tithes of Israel for a possession, for the ministry wherewith they serve Me in the Tabernacle." Now the clergy are the successors of the sons of Levi in the New Testament. Therefore tithes are due to the clergy alone.
I answer that Two things have to be considered with regard to tithes: namely, the right to receive tithes, and the things given in the name of tithes. The right to receive tithes is a spiritual thing, for it arises from the debt in virtue of which the ministers of the altar have a right to the expenses of their ministry, and temporal things are due to those who sow spiritual things. This debt concerns none but the clergy who have care of souls, and so they alone are competent to have this right.On the other hand the things given in the name of tithes are material, wherefore they may come to be used by anyone, and thus it is that they fall into the hands of the laity.

Reply to Objection: 1. In the Old Law, as stated above (Article [1], ad 4), special tithes were earmarked for the assistance of the poor. But in the New Law the tithes are given to the clergy, not only for their own support, but also that the clergy may use them in assisting the poor. Hence they are not unnecessary; indeed Church property, oblations and first-fruits as well as tithes are all necessary for this same purpose.
2. Personal tithes are due to the church in whose parish a man dwells, while predial tithes seem more reasonably to belong to the church within whose bounds the land is situated. The law, however, prescribes that in this matter a custom that has obtained for a long time must be observed [*Cap. Cum sint, and Cap. Ad apostolicae, de Decimis, etc.]. The shepherd who grazes his flock at different seasons in two parishes, should pay tithe proportionately to both churches. And since the fruit of the flock is derived from the pasture, the tithe of the flock is due to the church in whose lands the flock grazes, rather than to the church on whose land the fold is situated.
3. Just as the Church can hand over to a layman the things she receives under the title of tithe, so too can she allow him to receive tithes that are yet to be paid, the right of receiving being reserved to the ministers of the Church. The motive may be either the need of the Church, as when tithes are due to certain soldiers through being granted to them in fee by the Church, or it may be the succoring of the poor; thus certain tithes have been granted by way of alms to certain lay religious, or to those that have no care of souls. Some religious, however, are competent to receive tithes, because they have care of souls.

Whether the clergy also are bound to pay tithes?

Objection: 1. It would seem that clerics also are bound to pay tithes. By common law [*Cap. Cum homines, de Decimis, etc.] the parish church should receive the tithes on the lands which are in its territory. Now it happens sometimes that the clergy have certain lands of their own on the territory of some parish church, or that one church has ecclesiastical property on the territory of another. Therefore it would seem that the clergy are bound to pay predial tithes.
2. Further, some religious are clerics; and yet they are bound to pay tithes to churches on account of the lands which they cultivate even with their own hands [*Cap. Ex parte, and Cap. Nuper.]. Therefore it would seem that the clergy are not immune from the payment of tithes.
3. Further, in the eighteenth chapter of Numbers (Nb 26,28), it is prescribed not only that the Levites should receive tithes from the people, but also that they should themselves pay tithes to the high-priest. Therefore the clergy are bound to pay tithes to the Sovereign Pontiff, no less than the laity are bound to pay tithes to the clergy.
4. Further, tithes should serve not only for the support of the clergy, but also for the assistance of the poor. Therefore, if the clergy are exempt from paying tithes, so too are the poor. Yet the latter is not true. Therefore the former is false.

On the contrary A decretal of Pope Paschal [*Paschal II] says: "It is a new form of exaction when the clergy demand tithes from the clergy" [*Cap. Novum genus, de Decimis, etc.].
I answer that The cause of giving cannot be the cause of receiving, as neither can the cause of action be the cause of passion; yet it happens that one and the same person is giver and receiver, even as agent and patient, on account of different causes and from different points of view. Now tithes are due to the clergy as being ministers of the altar and sowers of spiritual things among the people. Wherefore those members of the clergy as such, i.e. as having ecclesiastical property, are not bound to pay tithes; whereas from some other cause through holding property in their own right, either by inheriting it from their kindred, or by purchase, or in any other similar manner, they are bound to the payment of tithes.

Reply to Objection: 1. Hence the Reply to the First Objection is clear, because the clergy like anyone else are bound to pay tithes on their own lands to the parish church, even though they be the clergy of that same church, because to possess a thing as one's private property is not the same as possessing it in common. But church lands are not tithable, even though they be within the boundaries of another parish.
2. Religious who are clerics, if they have care of souls, and dispense spiritual things to the people, are not bound to pay tithes, but they may receive them. Another reason applies to other religious, who though clerics do not dispense spiritual things to the people; for according to the ordinary law they are bound to pay tithes, but they are somewhat exempt by reason of various concessions granted by the Apostolic See [*Cap. Ex multiplici, Ex parte, and Ad audientiam, de Decimis, etc.].
3. In the Old Law first-fruits were due to the priests, and tithes to the Levites; and since the Levites were below the priests, the Lord commanded that the former should pay the high-priest "the tenth part of the tenth" [*Nb 18,26] instead of first-fruits: wherefore for the same reason the clergy are bound now to pay tithes to the Sovereign Pontiff, if he demanded them. For natural reason dictates that he who has charge of the common estate of a multitude should be provided with all goods, so that he may be able to carry out whatever is necessary for the common welfare.
4. Tithes should be employed for the assistance of the poor, through the dispensation of the clergy. Hence the poor have no reason for accepting tithes, but they are bound to pay them.

SERVICE BY PROMISE (Question [88])


We must now consider vows, whereby something is promised to God. Under this head there are twelve points of inquiry:

(1) What is a vow?

(2) What is the matter of a vow?

(3) Of the obligation of vows;

(4) Of the use of taking vows;

(5) Of what virtue is it an act?

(6) Whether it is more meritorious to do a thing from a vow, than without a vow?

(7) Of the solemnizing of a vow;

(8) Whether those who are under another's power can take vows?

(9) Whether children may be bound by vow to enter religion?

(10) Whether a vow is subject to dispensation or commutation?

(11) Whether a dispensation can be granted in a solemn vow of continence?

(12) Whether the authority of a superior is required in a dispensation from a vow?

Whether a vow consists in a mere purpose of the will?

Objection: 1. It would seem that a vow consists in nothing but a purpose of the will. According to some [*William of Auxerre, Sum. Aur. III, xxviii, qu. 1; Albertus Magnus, Sent. iv, D, 38], "a vow is a conception of a good purpose after a firm deliberation of the mind, whereby a man binds himself before God to do or not to do a certain thing." But the conception of a good purpose and so forth, may consist in a mere movement of the will. Therefore a vow consists in a mere purpose of the will.
2. Further, the very word vow seems to be derived from "voluntas" [will], for one is said to do a thing "proprio voto" [by one's own vow] when one does it voluntarily. Now to "purpose" is an act of the will, while to "promise" is an act of the reason. Therefore a vow consists in a mere act of the will.
3. Further, our Lord said (Lc 9,62): "No man putting his hand to the plough, and looking back, is fit for the kingdom of God." Now from the very fact that a man has a purpose of doing good, he puts his hand to the plough. Consequently, if he look back by desisting from his good purpose, he is not fit for the kingdom of God. Therefore by a mere good purpose a man is bound before God, even without making a promise; and consequently it would seem that a vow consists in a mere purpose of the will.

On the contrary It is written (Qo 5,3): "If thou hast vowed anything to God, defer not to pay it, for an unfaithful and foolish promise displeaseth Him." Therefore to vow is to promise, and a vow is a promise.
I answer that A vow denotes a binding to do or omit some particular thing. Now one man binds himself to another by means of a promise, which is an act of the reason to which faculty it belongs to direct. For just as a man by commanding or praying, directs, in a fashion, what others are to do for him, so by promising he directs what he himself is to do for another. Now a promise between man and man can only be expressed in words or any other outward signs; whereas a promise can be made to God by the mere inward thought, since according to 1S 16,7, "Man seeth those things that appear, but the Lord beholdeth the heart." Yet we express words outwardly sometimes, either to arouse ourselves, as was stated above with regard to prayer (Question [83], Article [12]), or to call others to witness, so that one may refrain from breaking the vow, not only through fear of God, but also through respect of men. Now a promise is the outcome from a purpose of doing something: and a purpose presupposes deliberation, since it is the act of a deliberate will. Accordingly three things are essential to a vow: the first is deliberation. the second is a purpose of the will; and the third is a promise, wherein is completed the nature of a vow. Sometimes, however, two other things are added as a sort of confirmation of the vow, namely, pronouncement by word of mouth, according to Ps 65,13, "I will pay Thee my vows which my lips have uttered"; and the witnessing of others. Hence the Master says (Sent. iv, D, 38) that a vow is "the witnessing of a spontaneous promise and ought to be made to God and about things relating to God": although the "witnessing" may strictly refer to the inward protestation.

Reply to Objection: 1. The conceiving of a good purpose is not confirmed by the deliberation of the mind, unless the deliberation lead to a promise.
2. Man's will moves the reason to promise something relating to things subject to his will, and a vow takes its name from the will forasmuch as it proceeds from the will as first mover.
3. He that puts his hand to the plough does something already; while he that merely purposes to do something does nothing so far. When, however, he promises, he already sets about doing, although he does not yet fulfil his promise: even so, he that puts his hand to the plough does not plough yet, nevertheless he stretches out his hand for the purpose of ploughing.

Whether a vow should always be about a better good?

Objection: 1. It would seem that a vow need not be always about a better good. A greater good is one that pertains to supererogation. But vows are not only about matters of supererogation, but also about matters of salvation: thus in Baptism men vow to renounce the devil and his pomps, and to keep the faith, as a gloss observes on Ps 75,12, "Vow ye, and pay to the Lord your God"; and Jacob vowed (Gn 28,21) that the Lord should be his God. Now this above all is necessary for salvation. Therefore vows are not only about a better good.
2. Further, Jephte is included among the saints (He 11,32). Yet he killed his innocent daughter on account of his vow (Jg 11). Since, then, the slaying of an innocent person is not a better good, but is in itself unlawful, it seems that a vow may be made not only about a better good, but also about something unlawful.
3. Further, things that tend to be harmful to the person, or that are quite useless, do not come under the head of a better good. Yet sometimes vows are made about immoderate vigils or fasts which tend to injure the person: and sometimes vows are about indifferent matters and such as are useful to no purpose. Therefore a vow is not always about a better good.

On the contrary It is written (Dt 23,22): "If thou wilt not promise thou shalt be without sin."
I answer that As stated above (Article [1]), a vow is a promise made to God. Now a promise is about something that one does voluntarily for someone else: since it would be not a promise but a threat to say that one would do something against someone. In like manner it would be futile to promise anyone something unacceptable to him. Wherefore, as every sin is against God, and since no work is acceptable to God unless it be virtuous, it follows that nothing unlawful or indifferent, but only some act of virtue, should be the matter of a vow. But as a vow denotes a voluntary promise, while necessity excludes voluntariness, whatever is absolutely necessary, whether to be or not to be, can nowise be the matter of a vow. For it would be foolish to vow that one would die or that one would not fly.On the other hand, if a thing be necessary. not absolutely but on the supposition of an end---for instance if salvation be unattainable without it---it may be the matter of a vow in so far as it is done voluntarily, but not in so far as there is a necessity for doing it. But that which is not necessary, neither absolutely, nor on the supposition of an end, is altogether voluntary, and therefore is most properly the matter of a vow. And this is said to be a greater good in comparison with that which is universally necessary for salvation. Therefore, properly speaking, a vow is said to be about a better good.

Reply to Objection: 1. Renouncing the devil's pomps and keeping the faith of Christ are the matter of baptismal vows, in so far as these things are done voluntarily, although they are necessary for salvation. The same answer applies to Jacob's vow: although it may also be explained that Jacob vowed that he would have the Lord for his God, by giving Him a special form of worship to which he was not bound, for instance by offering tithes and so forth as mentioned further on in the same passage.
2. Certain things are good, whatever be their result; such are acts of virtue, and these can be, absolutely speaking, the matter of a vow: some are evil, whatever their result may be; as those things which are sins in themselves, and these can nowise be the matter of a vow: while some, considered in themselves, are good, and as such may be the matter of a vow, yet they may have an evil result, in which case the vow must not be kept. It was thus with the vow of Jephte, who as related in Jg 11,30-31, "made a vow to the Lord, saying: If Thou wilt deliver the children of Ammon into my hands, whosoever shall first come forth out of the doors of my house, and shall meet me when I return in peace . . . the same will I offer a holocaust to the Lord." For this could have an evil result if, as indeed happened, he were to be met by some animal which it would be unlawful to sacrifice, such as an ass or a human being. Hence Jerome says [*Implicitly 1 Contra Jovin.: Comment. in Micheam vi, viii: Comment. in Jerem. vii. The quotation is from Peter Comestor, Hist. Scholast.]: "In vowing he was foolish, through lack of discretion, and in keeping his vow he was wicked." Yet it is premised (Jg 11,29) that "the Spirit of the Lord came upon him," because his faith and devotion, which moved him to make that vow, were from the Holy Ghost; and for this reason he is reckoned among the saints, as also by reason of the victory which he obtained, and because it is probable that he repented of his sinful deed, which nevertheless foreshadowed something good.
3. The mortification of one's own body, for instance by vigils and fasting, is not acceptable to God except in so far as it is an act of virtue; and this depends on its being done with due discretion, namely, that concupiscence be curbed without overburdening nature. on this condition such things may be the matter of a vow. Hence the Apostle after saying (Rm 12,1), "Present your bodies a living sacrifice, holy, pleasing to God," adds, "your reasonable service." Since, however, man is easily mistaken in judging of matters concerning himself, such vows as these are more fittingly kept or disregarded according to the judgment of a superior, yet so that, should a man find that without doubt he is seriously burdened by keeping such a vow, and should he be unable to appeal to his superior, he ought not to keep it. As to vows about vain and useless things they should be ridiculed rather than kept.

Whether all vows are binding?

Objection: 1. It would seem that vows are not all binding. For man needs things that are done by another, more than God does, since He has no need for our goods (Ps 15,2). Now according to the prescription of human laws [*Dig. L. xii, de pollicitat., i] a simple promise made to a man is not binding; and this seems to be prescribed on account of the changeableness of the human will. Much less binding therefore is a simple promise made to God, which we call a vow.
2. Further, no one is bound to do what is impossible. Now sometimes that which a man has vowed becomes impossible to him, either because it depends on another's decision, as when, for instance, a man vows to enter a monastery, the monks of which refuse to receive him: or on account of some defect arising, for instance when a woman vows virginity, and afterwards is deflowered; or when a man vows to give a sum of money, and afterwards loses it. Therefore a vow is not always binding.
3. Further, if a man is bound to pay something, he must do so at once. But a man is not bound to pay his vow at once, especially if it be taken under a condition to be fulfilled in the future. Therefore a vow is not always binding.

On the contrary It is written (Qo 5,3-4): "Whatsoever thou hast vowed, pay it; and it is much better not to vow, than after a vow not to perform the things promised."
I answer that For one to be accounted faithful one must keep one's promises. Wherefore, according to Augustine [*Ep. xxxii, 2: De Mendac. xx] faith takes its name "from a man's deed agreeing with his word" [*'Fides . . . fiunt dicta' Cicero gives the same etymology (De Offic. i, 7)]. Now man ought to be faithful to God above all, both on account of God's sovereignty, and on account of the favors he has received from God. Hence man is obliged before all to fulfill the vows he has made to God, since this is part of the fidelity he owes to God. On the other hand, the breaking of a vow is a kind of infidelity. Wherefore Solomon gives the reason why vows should be paid to God, because "an unfaithful . . . promise displeaseth Him" [*Qo 5,3].

Reply to Objection: 1. Honesty demands that a man should keep any promise he makes to another man, and this obligation is based on the natural law. But for a man to be under a civil obligation through a promise he has made, other conditions are requisite. And although God needs not our goods, we are under a very great obligation to Him: so that a vow made to Him is most binding.
2. If that which a man has vowed becomes impossible to him through any cause whatsoever, he must do what he can, so that he have at least a will ready to do what he can. Hence if a man has vowed to enter a monastery, he must endeavor to the best of his power to be received there. And if his intention was chiefly to bind himself to enter the religious life, so that, in consequence, he chose this particular form of religious life, or this place, as being most agreeable to him, he is bound, should he be unable to be received there, to enter the religious life elsewhere. But if his principal intention is to bind himself to this particular kind of religious life, or to this particular place, because the one or the other pleases him in some special way, he is not bound to enter another religious house, if they are unwilling to receive him into this particular one. on the other hand, if he be rendered incapable of fulfilling his vow through his own fault, he is bound over and above to do penance for his past fault: thus if a woman has vowed virginity and is afterwards violated, she is bound not only to observe what is in her power, namely, perpetual continency, but also to repent of what she has lost by sinning.
3. The obligation of a vow is caused by our own will and intention, wherefore it is written (Dt 23,23): "That which is once gone out of thy lips, thou shalt observe, and shalt do as thou hast promised to the Lord thy God, and hast spoken with thy own will and with thy own mouth." Wherefore if in taking a vow, it is one's intention and will to bind oneself to fulfil it at once, one is bound to fulfil it immediately. But if one intend to fulfil it at a certain time, or under a certain condition, one is not bound to immediate fulfilment. And yet one ought not to delay longer than one intended to bind oneself, for it is written (Dt 23,21): "When thou hast made a vow to the Lord thy God thou shalt not delay to pay it: because the Lord thy God will require it; and if thou delay, it shall be imputed to thee for a sin."

Whether it is expedient to take vows?

Objection: 1. It would seem that it is not expedient to take vows. It is not expedient to anyone to deprive himself of the good that God has given him. Now one of the greatest goods that God has given man is liberty whereof he seems to be deprived by the necessity implicated in a vow. Therefore it would seem inexpedient for man to take vows.
2. Further, no one should expose himself to danger. But whoever takes a vow exposes himself to danger, since that which, before taking a vow, he could omit without danger, becomes a source of danger to him if he should not fulfil it after taking the vow. Hence Augustine says (Ep. cxxvii, ad Arment. et Paulin.): "Since thou hast vowed, thou hast bound thyself, thou canst not do otherwise. If thou dost not what thou hast vowed thou wilt not be as thou wouldst have been hadst thou not vowed. For then thou wouldst have been less great, not less good: whereas now if thou breakest faith with God (which God forbid) thou art the more unhappy, as thou wouldst have been happier, hadst thou kept thy vow." Therefore it is not expedient to take vows.
3. Further, the Apostle says (1Co 4,16): "Be ye followers of me, as I also am of Christ." But we do not read that either Christ or the Apostles took any vows. Therefore it would seem inexpedient to take vows.

On the contrary It is written (Ps 75,12): "Vow ye and pay to the Lord your God."
I answer that As stated above (Articles [1],2), a vow is a promise made to God. Now one makes a promise to a man under one aspect, and to God under another. Because we promise something to a man for his own profit; since it profits him that we should be of service to him, and that we should at first assure him of the future fulfilment of that service: whereas we make promises to God not for His but for our own profit. Hence Augustine says (Ep. cxxvii, ad Arment. et Paulin.): "He is a kind and not a needy exactor, for he does not grow rich on our payments, but makes those who pay Him grow rich in Him." And just as what we give God is useful not to Him but to us, since "what is given Him is added to the giver," as Augustine says (Ep. cxxvii, ad Arment. et Paulin.), so also a promise whereby we vow something to God, does not conduce to His profit, nor does He need to be assured by us, but it conduces to our profit, in so far as by vowing we fix our wills immovably on that which it is expedient to do. Hence it is expedient to take vows.

Reply to Objection: 1. Even as one's liberty is not lessened by one being unable to sin, so, too, the necessity resulting from a will firmly fixed to good does not lessen the liberty, as instanced in God and the blessed. Such is the necessity implied by a vow, bearing a certain resemblance to the confirmation of the blessed. Hence, Augustine says (Ep. cxxvii, ad Arment. et Paulin.) that "happy is the necessity that compels us to do the better things."
2. When danger arises from the deed itself, this deed is not expedient, for instance that one cross a river by a tottering bridge: but if the danger arise through man's failure in the deed, the latter does not cease to be expedient: thus it is expedient to mount on horseback, though there be the danger of a fall from the horse: else it would behoove one to desist from all good things, that may become dangerous accidentally. Wherefore it is written (Qo 11,4): "He that observeth the wind shall not sow, and he that considereth the clouds shall never reap." Now a man incurs danger, not from the vow itself, but from his fault, when he changes his mind by breaking his vow. Hence, Augustine says (Ep. cxxvii, ad Arment. et Paulin.): "Repent not of thy vow: thou shouldst rather rejoice that thou canst no longer do what thou mightest lawfully have done to thy detriment."
3. It was incompetent for Christ, by His very nature, to take a vow, both because He was God, and because, as man, His will was firmly fixed on the good, since He was a "comprehensor." By a kind of similitude, however, He is represented as saying (Ps 21,26): "I will pay my vows in the sight of them that fear Him," when He is speaking of His body, which is the Church.The apostles are understood to have vowed things pertaining to the state of perfection when "they left all things and followed Christ."

Summa Th. II-II EN Qu.87 a.2