25 Nor have I undertaken that in the present discourse, as it more pertains to you, who hast laid open the hiding-places of the Priscillianists, so far as relates to their false and perverse dogmas; that they may not seem to have been in such sort investigated as if they were meet to be taught, not to be argued against. Make it therefore more your work that they be beaten down and laid low, as you have made it, that they should be betrayed and laid open; lest while we wish to get at the discovery of men practising falsehood, we allow the falsehoods themselves, as if insuperable, to stand their ground; when we ought rather even in the hearts of latent heretics to destroy falsehoods, than by sparing falsehoods to find out the deceivers who practise falsehood.
Moreover, among those dogmas of theirs which are to be subverted, is this which they dogmatize, namely, that in order to hide religion religious people ought to lie, to that degree that not only concerning other matters, not pertaining to doctrine of religion, but concerning religion itself, it is meet to lie, that it may not become exposed to aliens; to wit, that one may deny Christ, in order that one may in the midst of His enemies be in secret a Christian. This impious and nefarious dogma likewise, I beseech you, overthrow; to bolster up which they in their argumentations do gather from the Scriptures testimonies to make it appear that lies are not only to be pardoned and tolerated, but even honored.
To you therefore it pertains, in refuting that detestable sect, to show that those testimonies of Scripture are so to be received, that either you shall teach those to be no lies which are accounted to be such, if they be understood in that manner in which they ought to be understood; or, that those are not to be imitated which be manifestly lies; or in any wise at last, that concerning those matters at least which pertain to doctrine of religion, it is in no wise meet to tell a lie.
For thus are they truly from the very foundation overthrown, while that is overthrown wherein they lurk: that in that very matter they be judged least fit for us to follow, most fit to be shunned, in that they, for the hiding of their heresy, do profess themselves liars. This it is in them that must from the very first be assaulted, this which is, as it were, their fitting bulwark must with blows of Truth be battered and cast down. Nor must we afford them another lurking-place, which they had not, wherein they may take refuge, to wit, that being perhaps betrayed of them whom they have essayed to seduce but could not, they should say, “We only wanted to try them, because prudent Catholics have taught that to find out heretics it is right to do this.”
But it is necessary with somewhat more earnest be-speaking of your favor to say why this seems to me a tripartite method of disputing against those who want to apply the divine Scriptures as advocates of their lies; to wit, by showing that some which are there accounted to be lies, are not what they are accounted, if rightly understood; next, that if there be there any manifest lies, they are not meet to be imitated; thirdly, contrary to all opinions of all persons who think it pertains to the duty of a good man sometimes to lie, that it must in every way be held that in doctrine of religion there must in no wise a lie be told. For these are the three things to follow up which I shortly before recommended, and in some sort enjoined you.
Source: To Consentius: Against Lying (New Advent)