Summa Th. II-II EN Qu.110 a.4

Whether every lie is a mortal sin?

Objection: 1. It seems that every lie is a mortal sin. For it is written (Ps 6,7): "Thou wilt destroy all that speak a lie," and (Sg 1,11): "The mouth that belieth killeth the soul." Now mortal sin alone causes destruction and death of the soul. Therefore every lie is a mortal sin.
2. Further, whatever is against a precept of the decalogue is a mortal sin. Now lying is against this precept of the decalogue: "Thou shalt not bear false witness." Therefore every lie is a mortal sin.
3. Further, Augustine says (De Doctr. Christ. i, 36): "Every liar breaks his faith in lying, since forsooth he wishes the person to whom he lies to have faith in him, and yet he does not keep faith with him, when he lies to him: and whoever breaks his faith is guilty of iniquity." Now no one is said to break his faith or "to be guilty of iniquity," for a venial sin. Therefore no lie is a venial sin.
4. Further, the eternal reward is not lost save for a mortal sin. Now, for a lie the eternal reward was lost, being exchanged for a temporal meed. For Gregory says (Moral. xviii) that "we learn from the reward of the midwives what the sin of lying deserves: since the reward which they deserved for their kindness, and which they might have received in eternal life, dwindled into a temporal meed on account of the lie of which they were guilty." Therefore even an officious lie, such as was that of the midwives, which seemingly is the least of lies, is a mortal sin.
5. Further, Augustine says (Lib. De Mend. xvii) that "it is a precept of perfection, not only not to lie at all, but not even to wish to lie." Now it is a mortal sin to act against a precept. Therefore every lie of the perfect is a mortal sin: and consequently so also is a lie told by anyone else, otherwise the perfect would be worse off than others.

On the contrary Augustine says on Ps 5,7, "Thou wilt destroy," etc.: "There are two kinds of lie, that are not grievously sinful yet are not devoid of sin, when we lie either in joking, or for the sake of our neighbor's good." But every mortal sin is grievous. Therefore jocose and officious lies are not mortal sins.
I answer that A mortal sin is, properly speaking, one that is contrary to charity whereby the soul lives in union with God, as stated above (Question [24], Article [12]; Question [35], Article [3]). Now a lie may be contrary to charity in three ways: first, in itself; secondly, in respect of the evil intended; thirdly, accidentally.A lie may be in itself contrary to charity by reason of its false signification. For if this be about divine things, it is contrary to the charity of God, whose truth one hides or corrupts by such a lie; so that a lie of this kind is opposed not only to the virtue of charity, but also to the virtues of faith and religion: wherefore it is a most grievous and a mortal sin. If, however, the false signification be about something the knowledge of which affects a man's good, for instance if it pertain to the perfection of science or to moral conduct, a lie of this description inflicts an injury on one's neighbor, since it causes him to have a false opinion, wherefore it is contrary to charity, as regards the love of our neighbor, and consequently is a mortal sin. On the other hand, if the false opinion engendered by the lie be about some matter the knowledge of which is of no consequence, then the lie in question does no harm to one's neighbor; for instance, if a person be deceived as to some contingent particulars that do not concern him. Wherefore a lie of this kind, considered in itself, is not a mortal sin.As regards the end in view, a lie may be contrary to charity, through being told with the purpose of injuring God, and this is always a mortal sin, for it is opposed to religion; or in order to injure one's neighbor, in his person, his possessions or his good name, and this also is a mortal sin, since it is a mortal sin to injure one's neighbor, and one sins mortally if one has merely the intention of committing a mortal sin. But if the end intended be not contrary to charity, neither will the lie, considered under this aspect, be a mortal sin, as in the case of a jocose lie, where some little pleasure is intended, or in an officious lie, where the good also of one's neighbor is intended. Accidentally a lie may be contrary to charity by reason of scandal or any other injury resulting therefrom: and thus again it will be a mortal sin, for instance if a man were not deterred through scandal from lying publicly.

Reply to Objection: 1. The passages quoted refer to the mischievous lie, as a gloss explains the words of Ps 5,7, "Thou wilt destroy all that speak a lie."
2. Since all the precepts of the decalogue are directed to the love of God and our neighbor, as stated above (Question [44], Article [1], ad 3; I-II 100,5, ad 1), a lie is contrary to a precept of the decalogue, in so far as it is contrary to the love of God and our neighbor. Hence it is expressly forbidden to bear false witness against our neighbor.
3. Even a venial sin can be called "iniquity" in a broad sense, in so far as it is beside the equity of justice; wherefore it is written (1Jn 3,4): "Every sin is iniquity [*Vulg.: 'And sin is iniquity.']." It is in this sense that Augustine is speaking.
4. The lie of the midwives may be considered in two ways. First as regards their feeling of kindliness towards the Jews, and their reverence and fear of God, for which their virtuous disposition is commended. For this an eternal reward is due. Wherefore Jerome (in his exposition of Is 65,21, 'And they shall build houses') explains that God "built them spiritual houses." Secondly, it may be considered with regard to the external act of lying. For thereby they could merit, not indeed eternal reward, but perhaps some temporal meed, the deserving of which was not inconsistent with the deformity of their lie, though this was inconsistent with their meriting an eternal reward. It is in this sense that we must understand the words of Gregory, and not that they merited by that lie to lose the eternal reward as though they had already merited it by their preceding kindliness, as the objection understands the words to mean.
5. Some say that for the perfect every lie is a mortal sin. But this assertion is unreasonable. For no circumstance causes a sin to be infinitely more grievous unless it transfers it to another species. Now a circumstance of person does not transfer a sin to another species, except perhaps by reason of something annexed to that person, for instance if it be against his vow: and this cannot apply to an officious or jocose lie. Wherefore an officious or a jocose lie is not a mortal sin in perfect men, except perhaps accidentally on account of scandal. We may take in this sense the saying of Augustine that "it is a precept of perfection not only not to lie at all, but not even to wish to lie": although Augustine says this not positively but dubiously, for he begins by saying: "Unless perhaps it is a precept," etc. Nor does it matter that they are placed in a position to safeguard the truth: because they are bound to safeguard the truth by virtue of their office in judging or teaching, and if they lie in these matters their lie will be a mortal sin: but it does not follow that they sin mortally when they lie in other matters.


In due sequence we must consider dissimulation and hypocrisy. Under this head there are four points of inquiry:

(1) Whether all dissimulation is a sin?

(2) Whether hypocrisy is dissimulation?

(3) Whether it is opposed to truth?

(4) Whether it is a mortal sin?

Whether all dissimulation is a sin?

Objection: 1. It seems that not all dissimulation is a sin. For it is written (Lc 24,28) that our Lord "pretended [Douay: 'made as though'] he would go farther"; and Ambrose in his book on the Patriarchs (De Abraham i) says of Abraham that he "spoke craftily to his servants, when he said" (Gn 22,5): "I and the boy will go with speed as far as yonder, and after we have worshipped, will return to you." Now to pretend and to speak craftily savor of dissimulation: and yet it is not to be said that there was sin in Christ or Abraham. Therefore not all dissimulation is a sin.
2. Further, no sin is profitable. But according to Jerome, in his commentary on Ga 2,11, "When Peter [Vulg.: 'Cephas'] was come to Antioch:---The example of Jehu, king of Israel, who slew the priest of Baal, pretending that he desired to worship idols, should teach us that dissimulation is useful and sometimes to be employed"; and David "changed his countenance before" Achis, king of Geth (1S 21,13). Therefore not all dissimulation is a sin.
3. Further, good is contrary to evil. Therefore if it is evil to simulate good, it is good to simulate evil.
4. Further, it is written in condemnation of certain people (Is 3,9): "They have proclaimed abroad their sin as Sodom, and they have not hid it." Now it pertains to dissimulation to hide one's sin. Therefore it is reprehensible sometimes not to simulate. But it is never reprehensible to avoid sin. Therefore dissimulation is not a sin.

On the contrary A gloss on Is 16,14, "In three years," etc., says: "Of the two evils it is less to sin openly than to simulate holiness." But to sin openly is always a sin. Therefore dissimulation is always a sin.
I answer that As stated above (Question [109], Article [3]; Question [110], Article [1]), it belongs to the virtue of truth to show oneself outwardly by outward signs to be such as one is. Now outward signs are not only words, but also deeds. Accordingly just as it is contrary to truth to signify by words something different from that which is in one's mind, so also is it contrary to truth to employ signs of deeds or things to signify the contrary of what is in oneself, and this is what is properly denoted by dissimulation. Consequently dissimulation is properly a lie told by the signs of outward deeds. Now it matters not whether one lie in word or in any other way, as stated above (Question [110], Article [1], Objection [2]). Wherefore, since every lie is a sin, as stated above (Question [110], Article [3]), it follows that also all dissimulation is a sin.

Reply to Objection: 1. As Augustine says (De Questions. Evang. ii), "To pretend is not always a lie: but only when the pretense has no signification, then it is a lie. When, however, our pretense refers to some signification, there is no lie, but a representation of the truth." And he cites figures of speech as an example, where a thing is "pretended," for we do not mean it to be taken literally but as a figure of something else that we wish to say. In this way our Lord "pretended He would go farther," because He acted as if wishing to go farther; in order to signify something figuratively either because He was far from their faith, according to Gregory (Hom. xxiii in ); or, as Augustine says (De Questions. Evang. ii), because, "as He was about to go farther away from them by ascending into heaven, He was, so to speak, held back on earth by their hospitality."Abraham also spoke figuratively. Wherefore Ambrose (De Abraham i) says that Abraham "foretold what he knew not": for he intended to return alone after sacrificing his son: but by his mouth the Lord expressed what He was about to do. It is evident therefore that neither dissembled.
2. Jerome employs the term "simulation" in a broad sense for any kind of pretense. David's change of countenance was a figurative pretense, as a gloss observes in commenting on the title of Ps 33, "I will bless the Lord at all times." There is no need to excuse Jehu's dissimulation from sin or lie, because he was a wicked man, since he departed not from the idolatry of Jeroboam (2R 10,29 2R 10,31). And yet he is praised withal and received an earthly reward from God, not for his dissimulation, but for his zeal in destroying the worship of Baal.
3. Some say that no one may pretend to be wicked, because no one pretends to be wicked by doing good deeds, and if he do evil deeds, he is evil. But this argument proves nothing. Because a man might pretend to be evil, by doing what is not evil in itself but has some appearance of evil: and nevertheless this dissimulation is evil, both because it is a lie, and because it gives scandal; and although he is wicked on this account, yet his wickedness is not the wickedness he simulates. And because dissimulation is evil in itself, its sinfulness is not derived from the thing simulated, whether this be good or evil.
4. Just as a man lies when he signifies by word that which he is not, yet lies not when he refrains from saying what he is, for this is sometimes lawful; so also does a man dissemble, when by outward signs of deeds or things he signifies that which he is not, yet he dissembles not if he omits to signify what he is. Hence one may hide one's sin without being guilty of dissimulation. It is thus that we must understand the saying of Jerome on the words of Isaias 3:9, that the "second remedy after shipwreck is to hide one's sin," lest, to wit, others be scandalized thereby.

Whether hypocrisy is the same as dissimulation?

Objection: 1. It seems that hypocrisy is not the same as dissimulation. For dissimulation consists in lying by deeds. But there may be hypocrisy in showing outwardly what one does inwardly, according to Mt 6,2, "When thou dost an alms-deed sound not a trumpet before thee, as the hypocrites do." Therefore hypocrisy is not the same as dissimulation.
2. Further, Gregory says (Moral. xxxi, 7): "Some there are who wear the habit of holiness, yet are unable to attain the merit of perfection. We must by no means deem these to have joined the ranks of the hypocrites, since it is one thing to sin from weakness, and another to sin from malice." Now those who wear the habit of holiness, without attaining the merit of perfection, are dissemblers, since the outward habit signifies works of perfection. Therefore dissimulation is not the same as hypocrisy.
3. Further, hypocrisy consists in the mere intention. For our Lord says of hypocrites (Mt 23,5) that "all their works they do for to be seen of men": and Gregory says (Moral. xxxi, 7) that "they never consider what it is that they do, but how by their every action they may please men." But dissimulation consists, not in the mere intention, but in the outward action: wherefore a gloss on Jb 36,13, "Dissemblers and crafty men prove the wrath of God," says that "the dissembler simulates one thing and does another: he pretends chastity, and delights in lewdness, he makes a show of poverty and fills his purse." Therefore hypocrisy is not the same as dissimulation.

On the contrary Isidore says (Etym. x): "'Hypocrite' is a Greek word corresponding to the Latin 'simulator,' for whereas he is evil within," he "shows himself outwardly as being good; (hypo) denoting falsehood, and (krisis), judgment."
I answer that As Isidore says (Etym. x), "the word hypocrite is derived from the appearance of those who come on to the stage with a disguised face, by changing the color of their complexion, so as to imitate the complexion of the person they simulate, at one time under the guise of a man, at another under the guise of a woman, so as to deceive the people in their acting." Hence Augustine says (De Serm. Dom. ii) that "just as hypocrites by simulating other persons act the parts of those they are not (since he that acts the part of Agamemnon is not that man himself but pretends to be), so too in the Church and in every department of human life, whoever wishes to seem what he is not is a hypocrite: for he pretends to be just without being so in reality."We must conclude, therefore, that hypocrisy is dissimulation, not, however, any form of dissimulation, but only when one person simulates another, as when a sinner simulates the person of a just man.

Reply to Objection: 1. The outward deed is a natural sign of the intention. Accordingly when a man does good works pertaining by their genus to the service of God, and seeks by their means to please, not God but man, he simulates a right intention which he has not. Wherefore Gregory says (Moral.) that "hypocrites make God's interests subservient to worldly purposes, since by making a show of saintly conduct they seek, not to turn men to God, but to draw to themselves the applause of their approval:" and so they make a lying pretense of having a good intention, which they have not, although they do not pretend to do a good deed without doing it.
2. The habit of holiness, for instance the religious or the clerical habit, signifies a state whereby one is bound to perform works of perfection. And so when a man puts on the habit of holiness, with the intention of entering the state of perfection, if he fail through weakness, he is not a dissembler or a hypocrite, because he is not bound to disclose his sin by laying aside the habit of holiness. If, however, he were to put on the habit of holiness in order to make a show of righteousness, he would be a hypocrite and a dissembler.
3. In dissimulation, as in a lie, there are two things: one by way of sign, the other by way of thing signified. Accordingly the evil intention in hypocrisy is considered as a thing signified, which does not tally with the sign: and the outward words, or deeds, or any sensible objects are considered in every dissimulation and lie as a sign.

Whether hypocrisy is contrary to the virtue of truth?

Objection: 1. It seems that hypocrisy is not contrary to the virtue of truth. For in dissimulation or hypocrisy there is a sign and a thing signified. Now with regard to neither of these does it seem to be opposed to any special virtue: for a hypocrite simulates any virtue, and by means of any virtuous deeds, such as fasting, prayer and alms deeds, as stated in Mt 6,1-18. Therefore hypocrisy is not specially opposed to the virtue of truth.
2. Further, all dissimulation seems to proceed from guile, wherefore it is opposed to simplicity. Now guile is opposed to prudence as above stated (Question [55], Article [4]). Therefore, hypocrisy which is dissimulation is not opposed to truth, but rather to prudence or simplicity.
3. Further, the species of moral acts is taken from their end. Now the end of hypocrisy is the acquisition of gain or vainglory: wherefore a gloss on Jb 27,8, "What is the hope of the hypocrite, if through covetousness he take by violence," says: "A hypocrite or, as the Latin has it, a dissimulator, is a covetous thief: for through desire of being honored for holiness, though guilty of wickedness, he steals praise for a life which is not his." [*The quotation is from St. Gregory's Moralia, Bk XVIII.] Therefore since covetousness or vainglory is not directly opposed to truth, it seems that neither is hypocrisy or dissimulation.

On the contrary All dissimulation is a lie, as stated above (Article [1]). Now a lie is directly opposed to truth. Therefore dissimulation or hypocrisy is also.
I answer that According to the Philosopher (Metaph. text. 13, 24, x), "contrariety is opposition as regards form," i.e. the specific form. Accordingly we must reply that dissimulation or hypocrisy may be opposed to a virtue in two ways, in one way directly, in another way indirectly. Its direct opposition or contrariety is to be considered with regard to the very species of the act, and this species depends on that act's proper object. Wherefore since hypocrisy is a kind of dissimulation, whereby a man simulates a character which is not his, as stated in the preceding article, it follows that it is directly opposed to truth whereby a man shows himself in life and speech to be what he is, as stated in Ethic. iv, 7.The indirect opposition or contrariety of hypocrisy may be considered in relation to any accident, for instance a remote end, or an instrument of action, or anything else of that kind.

Reply to Objection: 1. The hypocrite in simulating a virtue regards it as his end, not in respect of its existence, as though he wished to have it, but in respect of appearance, since he wishes to seem to have it. Hence his hypocrisy is not opposed to that virtue, but to truth, inasmuch as he wishes to deceive men with regard to that virtue. And he performs acts of that virtue, not as intending them for their own sake, but instrumentally, as signs of that virtue, wherefore his hypocrisy has not, on that account, a direct opposition to that virtue.
2. As stated above (Question [55], Articles [3],4,5), the vice directly opposed to prudence is cunning, to which it belongs to discover ways of achieving a purpose, that are apparent and not real: while it accomplishes that purpose, by guile in words, and by fraud in deeds: and it stands in relation to prudence, as guile and fraud to simplicity. Now guile and fraud are directed chiefly to deception, and sometimes secondarily to injury. Wherefore it belongs directly to simplicity to guard oneself from deception, and in this way the virtue of simplicity is the same as the virtue of truth as stated above (Question [109], Article [2], ad 4). There is, however, a mere logical difference between them, because by truth we mean the concordance between sign and thing signified, while simplicity indicates that one does not tend to different things, by intending one thing inwardly, and pretending another outwardly.
3. Gain or glory is the remote end of the dissembler as also of the liar. Hence it does not take its species from this end, but from the proximate end, which is to show oneself other than one is. Wherefore it sometimes happens to a man to pretend great things of himself, for no further purpose than the mere lust of hypocrisy, as the Philosopher says (Ethic. iv, 7), and as also we have said above with regard to lying (Question [110], Article [2]).

Whether hypocrisy is always a mortal sin?

Objection: 1. It seems that hypocrisy is always a mortal sin. For Jerome says on Is 16,14: "Of the two evils it is less to sin openly than to simulate holiness": and a gloss on Jb 1,21 [*St. Augustine on Ps 63,7], "As it hath pleased the Lord," etc., says that "pretended justice is no justice, but a twofold sin": and again a gloss on Lam. 4:6, "The iniquity . . . of my people is made greater than the sin of Sodom," says: "He deplores the sins of the soul that falls into hypocrisy, which is a greater iniquity than the sin of Sodom." Now the sins of Sodom are mortal sin. Therefore hypocrisy is always a mortal sin.
2. Further, Gregory says (Moral. xxxi, 8) that hypocrites sin out of malice. But this is most grievous, for it pertains to the sin against the Holy Ghost. Therefore a hypocrite always sins mortally.
3. Further, no one deserves the anger of God and exclusion from seeing God, save on account of mortal sin. Now the anger of God is deserved through hypocrisy according to Jb 36,13, "Dissemblers and crafty men prove the wrath of God": and the hypocrite is excluded from seeing God, according to Jb 13,16, "No hypocrite shall come before His presence." Therefore hypocrisy is always a mortal sin.

On the contrary Hypocrisy is lying by deed since it is a kind of dissimulation. But it is not always a mortal sin to lie by deed. Neither therefore is all hypocrisy a mortal sin.Further, the intention of a hypocrite is to appear to be good. But this is not contrary to charity. Therefore hypocrisy is not of itself a mortal sin.Further, hypocrisy is born of vainglory, as Gregory says (Moral. xxxi, 17). But vainglory is not always a mortal sin. Neither therefore is hypocrisy.
I answer that There are two things in hypocrisy, lack of holiness, and simulation thereof. Accordingly if by a hypocrite we mean a person whose intention is directed to both the above, one, namely, who cares not to be holy but only to appear so, in which sense Sacred Scripture is wont to use the term, it is evident that hypocrisy is a mortal sin: for no one is entirely deprived of holiness save through mortal sin. But if by a hypocrite we mean one who intends to simulate holiness, which he lacks through mortal sin, then, although he is in mortal sin, whereby he is deprived of holiness, yet, in his case, the dissimulation itself is not always a mortal sin, but sometimes a venial sin. This will depend on the end in view; for if this be contrary to the love of God or of his neighbor, it will be a mortal sin: for instance if he were to simulate holiness in order to disseminate false doctrine, or that he may obtain ecclesiastical preferment, though unworthy, or that he may obtain any temporal good in which he fixes his end. If, however, the end intended be not contrary to charity, it will be a venial sin, as for instance when a man takes pleasure in the pretense itself: of such a man it is said in Ethic. iv, 7 that "he would seem to be vain rather than evil"; for the same applies to simulation as to a lie.It happens also sometimes that a man simulates the perfection of holiness which is not necessary for spiritual welfare. Simulation of this kind is neither a mortal sin always, nor is it always associated with mortal sin.

Reply to Objection: 1. This suffices for the Replies to the Objections.


We must now consider boasting and irony, which are parts of lying according to the Philosopher (Ethic. iv, 7). Under the first head, namely, boasting, there are two points of inquiry:

(1) To which virtue is it opposed?

(2) Whether it is a mortal sin?

Whether boasting is opposed to the virtue of truth?

Objection: 1. It seems that boasting is not opposed to the virtue of truth. For lying is opposed to truth. But it is possible to boast even without lying, as when a man makes a show of his own excellence. Thus it is written (Esther 1:3,4) that Assuerus "made a great feast . . . that he might show the riches of the glory" and "of his kingdom, and the greatness and boasting of his power." Therefore boasting is not opposed to the virtue of truth.
2. Further, boasting is reckoned by Gregory (Moral. xxiii, 4) to be one of the four species of pride, "when," to wit, "a man boasts of having what he has not." Hence it is written (Jr 48,29-30): "We have heard the pride of Moab, he is exceeding proud: his haughtiness, and his arrogancy, and his pride, and the loftiness of his heart. I know, saith the Lord, his boasting, and that the strength thereof is not according to it." Moreover, Gregory says (Moral. xxxi, 7) that boasting arises from vainglory. Now pride and vainglory are opposed to the virtue of humility. Therefore boasting is opposed, not to truth, but to humility.
3. Further, boasting seems to be occasioned by riches; wherefore it is written (Sg 5,8): "What hath pride profited us? or what advantage hath the boasting of riches brought us?" Now excess of riches seems to belong to the sin of covetousness, which is opposed to justice or liberality. Therefore boasting is not opposed to truth.

On the contrary The Philosopher says (Ethic. ii, 7; iv, 7), that boasting is opposed to truth.
I answer that "Jactantia" [boasting] seems properly to denote the uplifting of self by words: since if a man wishes to throw [jactare] a thing far away, he lifts it up high. And to uplift oneself, properly speaking, is to talk of oneself above oneself [*Or 'tall-talking' as we should say in English]. This happens in two ways. For sometimes a man speaks of himself, not above what he is in himself, but above that which he is esteemed by men to be: and this the Apostle declines to do when he says (2Co 12,6): "I forbear lest any man should think of me above that which he seeth in me, or anything he heareth of me." In another way a man uplifts himself in words, by speaking of himself above that which he is in reality. And since we should judge of things as they are in themselves, rather than as others deem them to be, it follows that boasting denotes more properly the uplifting of self above what one is in oneself, than the uplifting of self above what others think of one: although in either case it may be called boasting. Hence boasting properly so called is opposed to truth by way of excess.

Reply to Objection: 1. This argument takes boasting as exceeding men's opinion.
2. The sin of boasting may be considered in two ways. First, with regard to the species of the act, and thus it is opposed to truth; as stated (in the body of the article and Question [110], Article [2]). Secondly, with regard to its cause, from which more frequently though not always it arises: and thus it proceeds from pride as its inwardly moving and impelling cause. For when a man is uplifted inwardly by arrogance, it often results that outwardly he boasts of great things about himself; though sometimes a man takes to boasting, not from arrogance, but from some kind of vanity, and delights therein, because he is a boaster by habit. Hence arrogance, which is an uplifting of self above oneself, is a kind of pride; yet it is not the same as boasting, but is very often its cause. For this reason Gregory reckons boasting among the species of pride. Moreover, the boaster frequently aims at obtaining glory through his boasting, and so, according to Gregory, it arises from vainglory considered as its end.
3. Wealth also causes boasting, in two ways. First, as an occasional cause, inasmuch as a man prides himself on his riches. Hence (Pr 8,18) "riches" are significantly described as "proud" [Douay: 'glorious']. Secondly, as being the end of boasting, since according to Ethic. iv, 7, some boast, not only for the sake of glory, but also for the sake of gain. Such people invent stories about themselves, so as to make profit thereby; for instance, they pretend to be skilled in medicine, wisdom, or divination.

Whether boasting is a mortal sin?

Objection: 1. It seems that boasting is a mortal sin. For it is written (Pr 28,25): "He that boasteth, and puffeth himself, stirreth up quarrels." Now it is a mortal sin to stir up quarrels, since God hates those that sow discord, according to Pr 6,19. Therefore boasting is a mortal sin.
2. Further, whatever is forbidden in God's law is a mortal sin. Now a gloss on Si 6,2, "Extol not thyself in the thoughts of thy soul," says: "This is a prohibition of boasting and pride." Therefore boasting is a mortal sin.
3. Further, boasting is a kind of lie. But it is neither an officious nor a jocose lie. This is evident from the end of lying; for according to the Philosopher (Ethic. iv, 7), "the boaster pretends to something greater than he is, sometimes for no further purpose, sometimes for the sake of glory or honor, sometimes for the sake of money." Thus it is evident that it is neither an officious nor a jocose lie, and consequently it must be a mischievous lie. Therefore seemingly it is always a mortal sin.

On the contrary Boasting arises from vainglory, according to Gregory (Moral. xxxi, 17). Now vainglory is not always a mortal sin, but is sometimes a venial sin which only the very perfect avoid. For Gregory says (Moral. viii, 30) that "it belongs to the very perfect, by outward deeds so to seek the glory of their author, that they are not inwardly uplifted by the praise awarded them." Therefore boasting is not always a mortal sin.
I answer that As stated above (Question [110], Article [4]), a mortal sin is one that is contrary to charity. Accordingly boasting may be considered in two ways. First, in itself, as a lie, and thus it is sometimes a mortal, and sometimes a venial sin. It will be a mortal sin when a man boasts of that which is contrary to God's glory---thus it is said in the person of the king of Tyre (Ez 28,2): "Thy heart is lifted up, and thou hast said: I am God"---or contrary to the love of our neighbor, as when a man while boasting of himself breaks out into invectives against others, as told of the Pharisee who said (Lc 18,11): "I am not as the rest of men, extortioners, unjust, adulterers, as also is this publican." Sometimes it is a venial sin, when, to wit, a man boasts of things that are against neither God nor his neighbor. Secondly, it may be considered with regard to its cause, namely, pride, or the desire of gain or of vainglory: and then if it proceeds from pride or from such vainglory as is a mortal sin, then the boasting will also be a mortal sin: otherwise it will be a venial sin. Sometimes, however, a man breaks out into boasting through desire of gain, and for this very reason he would seem to be aiming at the deception and injury of his neighbor: wherefore boasting of this kind is more likely to be a mortal sin. Hence the Philosopher says (Ethic. iv, 7) that "a man who boasts for the sake of gain, is viler than one who boasts for the sake of glory or honor." Yet it is not always a mortal sin because the gain may be such as not to injure another man.

Reply to Objection: 1. To boast in order to stir quarrels is a mortal sin. But it happens sometimes that boasts are the cause of quarrels, not intentionally but accidentally: and consequently boasting will not be a mortal sin on that account.
2. This gloss speaks of boasting as arising from pride that is a mortal sin.
3. Boasting does not always involve a mischievous lie, but only where it is contrary to the love of God or our neighbor, either in itself or in its cause. That a man boast, through mere pleasure in boasting, is an inane thing to do, as the Philosopher remarks (Ethic. iv, 7): wherefore it amounts to a jocose lie. Unless perchance he were to prefer this to the love of God, so as to contemn God's commandments for the sake of boasting: for then it would be against the charity of God, in Whom alone ought our mind to rest as in its last end.To boast for the sake of glory or gain seen to involve an officious lie: provided it be do without injury to others, for then it would once become a mischievous lie.

Summa Th. II-II EN Qu.110 a.4