2 Do you not perceive how much this reasoning aids the very persons whom as great game we make ado to catch by our lies? For, as yourself hast shown, this is the sentiment of the Priscillianists to prove which, they apply testimonies from the Scriptures exhorting their followers to lie, as though by the examples of Patriarchs, Prophets, Apostles, Angels; not hesitating to add even the Lord Christ Himself; and deeming that they cannot otherwise prove their falsehood truthful, unless they pronounce Truth to be a liar.
It must be refuted, this; not imitated: nor ought we to be partners with the Priscillianists in that evil in which they are convicted to be worse than other heretics. For they alone, or at least they in the greatest degree, are found to make a dogma of lying for the purpose of hiding their truth, as they call it: and this so great evil therefore to esteem just, because they say that in the heart must be held that which is true, but with the mouth to utter unto aliens a false thing, is no sin; and that this is written, “Who speaks the truth in his heart:” as though this were enough for righteousness, even though a person do with his mouth speak a lie, when not his neighbor but a stranger is he that hears it.
On this account they think the Apostle Paul, when he had said, “Putting away lying, speak ye truth,” to have immediately added, “Every man with his neighbor, for we are members one of another.” Meaning, that with them who are not our neighbors in society of the truth, nor, so to say, our co-members, it is lawful and right to speak a lie.
Source: To Consentius: Against Lying (New Advent)