Objection 1. It seems that not every lie is a sin. For it is evident that the evangelists did not sin in the writing of the Gospel. Yet they seem to have told something false: since their accounts of the words of Christ and of others often differ from one another: wherefore seemingly one of them must have given an untrue account. Therefore not every lie is a sin.
Objection 2. Further, no one is rewarded by God for sin. But the midwives of Egypt were rewarded by God for a lie, for it is stated that "God built them houses" (Exodus 1:21). Therefore a lie is not a sin.
Objection 3. Further, the deeds of holy men are related in Sacred Writ that they may be a model of human life. But we read of certain very holy men that they lied. Thus (Genesis 12 and 20) we are told that Abraham said of his wife that she was his sister. Jacob also lied when he said that he was Esau, and yet he received a blessing (Genesis 27:27-29). Again, Judith is commended (Judith 15:10-11) although she lied to Holofernes. Therefore not every lie is a sin.
Objection 4. Further, one ought to choose the lesser evil in order to avoid the greater: even so a physician cuts off a limb, lest the whole body perish. Yet less harm is done by raising a false opinion in a person's mind, than by someone slaying or being slain. Therefore a man may lawfully lie, to save another from committing murder, or another from being killed.
Objection 5. Further, it is a lie not to fulfill what one has promised. Yet one is not bound to keep all one's promises: for Isidore says (Synonym. ii): "Break your faith when you have promised ill." Therefore not every lie is a sin.
Objection 6. Further, apparently a lie is a sin because thereby we deceive our neighbor: wherefore Augustine says (Lib. De Mend. xxi): "Whoever thinks that there is any kind of lie that is not a sin deceives himself shamefully, since he deems himself an honest man when he deceives others." Yet not every lie is a cause of deception, since no one is deceived by a jocose lie; seeing that lies of this kind are told, not with the intention of being believed, but merely for the sake of giving pleasure. Hence again we find hyperbolical expressions in Holy Writ. Therefore not every lie is a sin.
On the contrary, It is written (Sirach 7:14): "Be not willing to make any manner of lie."
I answer that, An action that is naturally evil in respect of its genus can by no means be good and lawful, since in order for an action to be good it must be right in every respect: because good results from a complete cause, while evil results from any single defect, as Dionysius asserts (Div. Nom. iv). Now a lie is evil in respect of its genus, since it is an action bearing on undue matter. For as words are naturally signs of intellectual acts, it is unnatural and undue for anyone to signify by words something that is not in his mind. Hence the Philosopher says (Ethic. iv, 7) that "lying is in itself evil and to be shunned, while truthfulness is good and worthy of praise." Therefore every lie is a sin, as also Augustine declares (Contra Mend. i).
Reply to Objection 1. It is unlawful to hold that any false assertion is contained either in the Gospel or in any canonical Scripture, or that the writers thereof have told untruths, because faith would be deprived of its certitude which is based on the authority of Holy Writ. That the words of certain people are variously reported in the Gospel and other sacred writings does not constitute a lie. Hence Augustine says (De Consens. Evang. ii): "He that has the wit to understand that in order to know the truth it is necessary to get at the sense, will conclude that he must not be the least troubled, no matter by what words that sense is expressed." Hence it is evident, as he adds (De Consens. Evang. ii), that "we must not judge that someone is lying, if several persons fail to describe in the same way and in the same words a thing which they remember to have seen or heard."
Reply to Objection 2. The midwives were rewarded, not for their lie, but for their fear of God, and for their good-will, which latter led them to tell a lie. Hence it is expressly stated (Exodus 2:21): "And because the midwives feared God, He built them houses." But the subsequent lie was not meritorious.
Reply to Objection 3. In Holy Writ, as Augustine observes (Lib. De Mend. v), the deeds of certain persons are related as examples of perfect virtue: and we must not believe that such persons were liars. If, however, any of their statements appear to be untruthful, we must understand such statements to have been figurative and prophetic. Hence Augustine says (Lib. De Mend. v): "We must believe that whatever is related of those who, in prophetical times, are mentioned as being worthy of credit, was done and said by them prophetically." As to Abraham "when he said that Sara was his sister, he wished to hide the truth, not to tell a lie, for she is called his sister since she was the daughter of his father," Augustine says (QQ. Super. Gen. xxvi; Contra Mend. x; Contra Faust. xxii). Wherefore Abraham himself said (Genesis 20:12): "She is truly my sister, the daughter of my father, and not the daughter of my mother," being related to him on his father's side. Jacob's assertion that he was Esau, Isaac's first-born, was spoken in a mystical sense, because, to wit, the latter's birthright was due to him by right: and he made use of this mode of speech being moved by the spirit of prophecy, in order to signify a mystery, namely, that the younger people, i.e. the Gentiles, should supplant the first-born, i.e. the Jews.
Some, however, are commended in the Scriptures, not on account of perfect virtue, but for a certain virtuous disposition, seeing that it was owing to some praiseworthy sentiment that they were moved to do certain undue things. It is thus that Judith is praised, not for lying to Holofernes, but for her desire to save the people, to which end she exposed herself to danger. And yet one might also say that her words contain truth in some mystical sense.
Reply to Objection 4. A lie is sinful not only because it injures one's neighbor, but also on account of its inordinateness, as stated above in this Article. Now it is not allowed to make use of anything inordinate in order to ward off injury or defects from another: as neither is it lawful to steal in order to give an alms, except perhaps in a case of necessity when all things are common. Therefore it is not lawful to tell a lie in order to deliver another from any danger whatever. Nevertheless it is lawful to hide the truth prudently, by keeping it back, as Augustine says (Contra Mend. x).
Reply to Objection 5. A man does not lie, so long as he has a mind to do what he promises, because he does not speak contrary to what he has in mind: but if he does not keep his promise, he seems to act without faith in changing his mind. He may, however, be excused for two reasons. First, if he has promised something evidently unlawful, because he sinned in promise, and did well to change his mind. Secondly, if circumstances have changed with regard to persons and the business in hand. For, as Seneca states (De Benef. iv), for a man to be bound to keep a promise, it is necessary for everything to remain unchanged: otherwise neither did he lie in promising--since he promised what he had in his mind, due circumstances being taken for granted--nor was he faithless in not keeping his promise, because circumstances are no longer the same. Hence the Apostle, though he did not go to Corinth, whither he had promised to go (2 Corinthians 1), did not lie, because obstacles had arisen which prevented him.
Reply to Objection 6. An action may be considered in two ways. First, in itself, secondly, with regard to the agent. Accordingly a jocose lie, from the very genus of the action, is of a nature to deceive; although in the intention of the speaker it is not told to deceive, nor does it deceive by the way it is told. Nor is there any similarity in the hyperbolical or any kind of figurative expressions, with which we meet in Holy Writ: because, as Augustine says (Lib. De Mend. v), "it is not a lie to do or say a thing figuratively: because every statement must be referred to the thing stated: and when a thing is done or said figuratively, it states what those to whom it is tendered understand it to signify."
Source: Summa Theologica (New Advent)