37 Add to this, (and here is cause to cry out more piteously,) that, if once we grant it to have been right for the saving of that sick man's life to tell him the lie, that his son was alive, then, little by little and by minute degrees, the evil so grows upon us, and by slight accesses to such a heap of wicked lies does it, in its almost imperceptible encroachments, at last come, that no place can ever be any where found on which this huge mischief, by smallest additions rising into boundless strength, might be resisted.
Wherefore, most providently is it written, “He that despises small things shall fall little by little.” Nay more: for these persons who are so enamored of this life, that they hesitate not to prefer it to truth, that a man may not die, say rather, that a man who must some time die may die somewhat later, would have us not only to lie, but even to swear fasely; to wit, that, lest the vain health of man should somewhat more quickly pass away, we should take the name of the Lord our God in vain!
And there are among them learned men who even fix rules, and set bounds when it is a duty, when not a duty, to commit perjury! O, where are you, fountains of tears? And what shall we do? Whither go? Where hide us from the ire of truth, if we not only neglect to shun lies, but dare moreover to teach perjuries? For look they well to it, who uphold and defend lying, what kind, or what kinds, of lying they shall delight to justify: at least in the worship of God let them grant that there must be no lying; at least let them keep themselves from perjuries and blasphemies; at least there, where God's name, where God as witness, where God's oath is interposed, where God's religion is the matter of discourse or colloquy, let none lie, none praise, none teach and enjoin, none justify a lie: of the other kinds of lies let him choose him out that which he accounts to be the mildest and most innocent kind of lying, he who will have it to be right to lie.
This I know, that even he who teaches that it is meet to tell lies, wishes to be thought to teach a truth. For if it be false which he teaches, who would care to give heed to false doctrine, in which both he deceives that teaches and he is deceived that learns? But if, in order that he may be able to find some disciple, he upholds that he teaches a truth when he teaches that it is meet to lie, how will that lie be of the truth, when the Apostle John reclaims, “No lie is of the truth?” It is therefore not true, that it is sometimes right to lie; and that which is not true to no man is at all to be persuaded.
Source: To Consentius: Against Lying (New Advent)