38 But infirmity pleads its part, and with favor of the crowds proclaims itself to have a cause invincible. Where it contradicts, and says, “What way is there among men, who without doubt by being deceived are turned aside from a deadly harm to others or themselves, to succor men in peril, if our affection as men may not incline us to lie?” If it will hear me patiently, this crowd of mortality, crowd of infirmity, I will say somewhat in answer on the behalf of truth.
Surely at the least pious, true, holy chastity is not otherwise than of the truth: and whoso acts against it, acts against truth. Why then, if otherwise it be not possible to succor men in peril, do I not also commit whoredom, which is therefore contrary to truth, for that it is contrary to chastity, and yet, to succor men in peril, do speak a lie which most openly is contrary to truth itself? Wherein has chastity so highly deserved at our hands, and truth offended us? When all chastity is of the truth, and not the body's but the mind's chastity is truth, yea, in the mind dwells even the body's chastity.
Lastly, as I shortly before said, and say again, whoever for the recommending and defending of any lie speaks against me, what speaks he, if he speaks not truth? Now if he is therefore to be heard because he speaks truth, how wishes he to make me, by speaking truth, a liar? How does lying take unto itself truth as its patroness? Or, is it for her own adversary that she conquers, that by herself she may be conquered? Who can bear this absurdity? In no wise therefore may we say, that they who assert that it is sometimes right to lie, in asserting that are truthful; lest, what is most absurd and foolish to believe, truth should teach us to be liars.
For what sort of thing is it, that no man learns of chastity that we may commit adultery; that we may offend God none learns of piety; that we may do any man harm, none learns of kindness; and that we may tell lies, we are to learn of truth! But then if this thing truth teaches not, it is not true; if not true, it is not meet to be learned; if not meet to be learned, never therefore is it meet to tell a lie.
Source: To Consentius: Against Lying (New Advent)