Objection 1. It seems that every lie is a mortal sin. For it is written (Psalm 6:7): "Thou wilt destroy all that speak a lie," and (Wisdom 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.
Objection 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.
Objection 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.
Objection 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.
Objection 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 Psalm 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 (24, 12; 35, 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 Psalm 5:7, "Thou wilt destroy all that speak a lie."
Reply to Objection 2. Since all the precepts of the decalogue are directed to the love of God and our neighbor, as stated above (44, 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.
Reply to Objection 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 (1 John 3:4): "Every sin is iniquity [Vulgate: 'And sin is iniquity.']." It is in this sense that Augustine is speaking.
Reply to Objection 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 Isaiah 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.
Reply to Objection 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.
Source: Summa Theologica (New Advent)