4 Of lies are many sorts, which indeed all, universally, we ought to hate. For there is no lie that is not contrary to truth. For, as light and darkness, piety and impiety, justice and iniquity, sin and right-doing, health and weakness, life and death, so are truth and a lie contrary the one to the other. Whence by how much we love the former, by so much ought we to hate the latter. Yet in truth there be some lies which to believe does no harm: although even by such sort of lie to wish to deceive, is hurtful to him that tells it, not to him that believes it.
As though, if that brother, the servant of God, Fronto, in the information which he gave you, should (though far be the thought!) say some things falsely; he would have hurt himself assuredly, not you, although you, without iniquity of yours, had believed all, upon his telling it. Because, whether those things did so take place or not so, yet they have not any thing, which if a person believe to have been so, though it were not so, he by the rule of truth and doctrine of eternal salvation should be judged worthy of blame.
Whereas, if a person tell a lie which if any believe he will be an heretic against the doctrine of Christ, by so much is he who tells the lie more hurtful, by how much he that believes it is more miserable. See then, what manner of thing it is, if against the doctrine of Christ we shall tell a lie which whoso believes shall perish, in order that we may catch the enemies of the same doctrine, to the end we may bring them to the truth, while we recede from it; nay rather, when we catch liars by lying, teach worse lies.
For it is one thing what they say when they lie, another when they are deceived. For, when they teach their heresy, they speak the things in which they are deceived; but when they say that they think what they do not think, or that they do not think what they do think, they say the things in which they lie. In that any believes them, what though he do not find them out, himself perishes not. For it is no receding from the catholic rule, if, when a heretic lyingly professes the catholic doctrines, one believes him to be a catholic: and therefore it is not pernicious to him; because he is mistaken in the mind of a man, of which, when latent, he cannot judge, not in the faith of God which it is his duty to keep safe planted within him.
Moreover, when they teach their heresy, whoso shall believe them, in thinking it truth, will be partaker, as of their error, so of their damnation. So it comes to pass, that when they fable their nefarious dogmas in which they are with deadly error deceived, then whoso believes them is lost: whereas when we preach catholic dogmas, in which we hold the right faith, then if he shall believe, that man is found, whoso was lost. But when, they being Priscillianists, do, in order that they may not betray their venom, lyingly give themselves out to be of us; whoever of us believes them, even while they escape detection, himself perseveres a Catholic: we on the other hand, if, in order to attain to the discovery of them, we falsely give ourselves out for Priscillianists, because we shall praise their dogmas as though they were our own, whoso shall believe the same, will either be confirmed among them, or will be transferred to them in the meantime straightway: but what the coming hour may bring forth, whether they shall be afterwards set free therefrom by us when speaking true things, who were deceived by us when speaking false; and whether they will be willing to hear one teaching whom they have thus experienced telling a lie,who can know for certain? Who can be ignorant that this is uncertain? Whence it is gathered, that it is more pernicious, or to speak more mildly, that it is more perilous for Catholics to lie that they may catch heretics, than for heretics to lie that they may not be found out by Catholics. Because, whoso believes Catholics when they tell a lie to tempt people, is either made or confirmed a heretic; but whoso believes heretics when they tell a lie to conceal themselves, does not cease to be a Catholic. But that this may become more plain, let us propose some cases by way of example, and from those writings in preference which you have sent me to read.
Source: To Consentius: Against Lying (New Advent)