Nicene and Post-Nicene Fathers, Vol. III:
Moral Treatises of St. Augustin: Section 6

6. It remains, then, that what the Priscillianists think, according to the nefarious falsity of their heresy, of God, of the soul, of the body, and the rest, we hesitate not with truthful pity to condemn; but what they think of the right of telling a lie to hide the truth is to be to us and them (which God forbid!) a common dogma. This is so great an evil, that even though this attempt of ours, whereby we desire by means of a lie to catch them and change them, should so prosper that we do catch and change them, there is no gain that can compensate the damage of making ourselves wrong with them in order to set them right. For through this lie shall both we be in that respect perverse, and they but half corrected; seeing that their thinking it right to tell a lie on behalf of the truth is a fault which we do not correct in them, because we have learned and do teach the same thing, and lay it down that it is fit to be done, in order that we may be able to attain to the amending of them. Whom yet we amend not, for their fault, with which they think right to hide the truth, we take not away, rather we make ourselves faulty when by such a fault we seek them; nor do we find how we can believe them, when converted, to whom, while perverted, we have lied; lest haply what was done to them that they might be caught, they do to us when caught; not only because to do it hath been their wont, but because in us also, to whom they come, they find the same.