The Enchiridion by St. Augustine
Chapter 22.--A Lie is Not Allowable, Even to Save Another from Injury.
But every lie must be called a sin, because not only when a man knows the truth, but even when, as a man may be, he is mistaken and deceived, it is his duty to say what he thinks in his heart, whether it be true, or whether he only think it to be true. But every liar says the opposite of what he thinks in his heart, with purpose to deceive. Now it is evident that speech was given to man, not that men might therewith deceive one another, but that one man might make known his thoughts to another. To use speech, then, for the purpose of deception, and not for its appointed end, is a sin. Nor are we to suppose that there is any lie that is not a sin, because it is sometimes possible, by telling a lie, to do service to another. For it is possible to do this by theft also, as when we steal from a rich man who never feels the loss, to give to a poor man who is sensibly benefited by what he gets. And the same can be said of adultery also, when, for instance, some woman appears likely to die of love unless we consent to her wishes, while if she lived she might purify herself by repentance; but yet no one will assert that on this account such an adultery is not a sin. And if we justly place so high a value upon chastity, what offense have we taken at truth, that, while no prospect of advantage to another will lead us to violate the former by adultery, we should be ready to violate the latter by lying? It cannot be denied that they have attained a very high standard of goodness who never lie except to save a man from injury; but in the case of men who have reached this standard, it is not the deceit, but their good intention, that is justly praised, and sometimes even rewarded. It is quite enough that the deception should be pardoned, without its being made an object of laudation, especially among the heirs of the new covenant, to whom it is said: |Let your communication be, Yea, yea; Nay, nay: for whatsoever is more than these cometh of evil.| And it is on account of this evil, which never ceases to creep in while we retain this mortal vesture, that the co-heirs of Christ themselves say, |Forgive us our debts.|