17 Wherefore, though there be indeed many ways in which latent heretics may be sought out, without vituperating the catholic faith or praising heretical impiety, yet if there were no other way at all of drawing out heretical impiety from its caverns, but that the catholic tongue should deviate from the straight path of truth; more tolerable were it that that should be hid, than that this should be precipitated; more tolerable that the foxes should lurk in their pits unseen, than for the sake of catching them the huntsmen should fall into the pit of blasphemy; more tolerable that the perfidy of Priscillianists should be covered with the veil of truth, than that the faith of Catholics, lest it should of lying Priscillianists be praised, should of believing Catholics be denied.
For if lies, not of whatsoever kind, but blasphemous lies, are therefore just because they are committed with intent to detect hidden heretics; it will be possible at that rate, if they be commuted with the same intention, that there should be chaste adulteries. For put the case that of a number of lewd Priscillianists, some woman should cast her eye upon a catholic Joseph, and promise him that she will betray their hidden retreats if she obtain from him that he lie with her, and it be certain that if he consent unto her she will make good her promise: shall we judge that it ought to be done?
Or shall we understand that by no means must such a price be paid in purchase of that kind of merchandise? Why then do we not rout out heretics, in order to their being caught, by the flesh committing lasciviousness in adultery, and yet think right to rout them out by a mouth committing fornication in blasphemy? For either it will be lawful to defend both the one and the other with equal reason, that these things be therefore said to be not unjust, because they were done with intention of finding out the unjust: or if sound doctrine wills not even for the sake of finding out heretics that we should have to do with unchaste women, albeit only in body, not in mind, assuredly not even for the sake of finding out heretics wills it that by us, albeit only in voice not in mind, either unclean heresy were preached, or the chaste Catholic Church blasphemed.
Because even the very sovereignty of the mind, to which every inferior motion of the man ought to be obedient, will not lack deserved opprobrium, when a thing is done that ought not to be done, whether by member or by word. Although even when it is done by word, it is done by member: because the tongue is a member, by which the word is made; nor is any deed of ours by any member brought to the birth unless it is first conceived in the heart; or rather being by our inwardly thinking upon and consenting unto it already brought to the birth, it is brought forth abroad in our doing of it, by a member. It is therefore no excusing the mind from the deed, when any thing is said to be done not after the purpose of the mind, which yet were not done, unless the mind decreed it to be done.
Source: To Consentius: Against Lying (New Advent)