12 “But,” you will say, “we more easily penetrate their concealment if we pretend to be ourselves what they are.” If this were lawful or expedient, Christ might have instructed his sheep that they should come clad in wolves' clothing to the wolves, and by the cheat of this artifice discover them: which He has not said, no, not when He foretold that He would send them forth in the midst of wolves. But you will say: “They needed not at that time to have inquisition made for them, being most manifest wolves; but their bite and savageness were to be endured.”
What, when foretelling later times, He said that ravening wolves would come in sheep's clothing? Was there not room there to give this advice and say, And do ye, that you may find them out, assume wolves' clothing, but within be ye sheep still? Not this says He: but when he had said, “Many will come to you in sheep's clothing, but within are ravening wolves;” He went on to say, not, By your lies, but, “By their fruits you shall know them.” By truth must we beware of, by truth must we take, by truth must we kill, lies.
Be it far from us, that the blasphemies of the ignorant we by wittingly blaspheming should overcome: far from us, that the evils of deceitful men we by imitating should guard against. For how shall we guard against them if in order to guard against them we shall have them? For if in order that he may be caught who blasphemes unwittingly, I shall blaspheme wittingly, worse is the thing I do than that which I catch. If in order that he may be found who denies Christ unwittingly, I shall deny Him wittingly, to his undoing will he follow me whom I shall so find, since in order that I may find him out, I first am undone.
Source: To Consentius: Against Lying (New Advent)