10 Ever, my brother, in such cases, it behooves with fear to recollect, “Whoso shall deny Me before men, I will deny him before My Father which is in heaven.” Or truly is it no denying of Christ before men, to deny Him before Priscillianists, that when they hide themselves, one may by a blasphemous lie strip them and catch them? But who doubts, I pray you, that Christ is denied, when so as He is in truth, we say that He is not; and so as the Priscillianist believes Him, we say that He is?
11. “But, hidden wolves,” you will say, “clad in sheep's clothing, and privily and grievously wasting the Lord's flock, can we no otherwise find out.” Whence then have the Priscillianists become known, ere this way of hunting for them with lies was ex-cogitated? Whence was their very author, more cunning doubtless, and therefore more covert, got at in his bed? Whence so many and so great persons made manifest and condemned, and the others innumerable partly corrected, partly as if corrected, and in the Church's compassion gathered into her fold? For many ways gives the Lord, when He has compassion, whereby we may come to the discovery of them: two of which are more happy than others; namely, that either they whom they have wished to seduce, or they whom they had already seduced, shall, when they repent and are converted, point them out. Which is more easily effected, if their nefarious error, not by lying tricks, but by truthful reasonings be overthrown. In the writing of which it behooves you to bestow your pains, since God has bestowed the gift that you can do this: which wholesome writings whereby their insane perversity is destroyed, becoming more and more known, and being by Catholics, whether prelates who speak in the congregations, or any studious men full of zeal for God, every where diffused, these will be holy nets in which they may be caught truthfully, not with lies hunted after. For so being taken, either, of their own accord, they will confess what they have been, and others whom they know to be of the evil fellowship they will either kindly correct, or mercifully betray. Or else, if they shall be ashamed to confess what with long-continued simulation they have concealed, by the hidden hand of God healing them shall they be made whole.
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)