42 It clearly appears then, all being discussed, that those testimonies of Scripture have none other meaning than that we must never at all tell a lie: seeing that not any examples of lies, worthy of imitation, are found in the manners and actions of the Saints, as regards those Scriptures which are referred to no figurative signification, such as is the history in the Acts of the Apostles. For all those sayings of our Lord in the Gospel, which to more ignorant minds seem lies, are figurative significations.
And as to what the Apostle says: “I am made all things to all men, that I might gain all;” the right understanding is, that he did this not by lying, but by sympathy; so that he dealt with them in liberating them with so great charity, as if he were himself in that evil from which he wished to make them whole. There must therefore be no lying in the doctrine of piety: it is a heinous wickedness, and the first sort of detestable lie. There must be no lying of the second sort; because no man must have a wrong done to him.
There must be no lying of the third sort; because we are not to consult any man's good to the injury of another. There must be no lying of the fourth sort, that is, for the lust of lying, which of itself is vicious. There must be no lying of the fifth sort, because not even the truth itself is to be uttered with the aim of men-pleasing, how much less a lie, which of itself, as a lie, is a foul thing? There must be no lying of the sixth sort; for it is not right that even the truth of testimony be corrupted for any man's temporal convenience and safety.
But unto eternal salvation none is to be led by aid of a lie. For not by the ill manners of them that convert him is he to be converted to good manners: because if it is meet to be done towards him, himself also ought when converted to do it toward others; and so is he converted not to good, but to ill manners, seeing that is held out to be imitated by him when converted, which was done unto him in converting him. Neither in the seventh sort must there be any lying; for it is meet that not any man's commodity or temporal welfare be preferred to the perfecting of faith.
Not even if any man is so ill moved by our right deeds as to become worse in his mind, and far more remote from piety, are right deeds therefore to be foregone: since what we are chiefly to hold is that whereunto we ought to call and invite them whom as our own selves we love; and with most courageous mind we must drink in that apostolic sentence: “To some we are a savor of life unto life, to others a savor of death unto death; and who is sufficient for these things?” Nor in the eighth sort must there be lying: because both among good things chastity of mind is greater than pudicity of body; and among evil things, that which ourselves do, than that which we suffer to be done.
In these eight kinds, however, a man sins less when he tells a lie, in proportion as he emerges to the eighth: more, in proportion as he diverges to the first. But whoso shall think there is any sort of lie that is not sin, will deceive himself foully, while he deems himself honest as a deceiver of other men.
Source: On Lying (New Advent)