Nicene and Post-Nicene Fathers: Series I/Volume III/Moral Treatises of St. Augustin/Against Lying/Section 14

14. Wherefore, that which is written, “Who speaketh the truth in his heart,” is not so to be taken, as if, truth being retained in the heart, in the mouth one may speak a lie. But the reason why it is said, is, because it is possible that a man may speak with his mouth a truth which profiteth him nothing, if he hold it not in his heart, that is, if what he speaketh, himself believe not; as the heretics, and, above all, these same Priscillianists do, when they do, not indeed believe the catholic faith, but yet speak it, that they may be believed to be of us. They speak therefore the truth in their mouth, not in their heart. On this account were they to be distinguished from him of whom it is written, “He that speaketh truth in his heart.” Now this truth the catholic as in his heart he speaketh, because so he believeth, so also in his mouth ought he, that so he may preach it; but against it, neither in heart nor in mouth have falsehood, that both with the heart he may believe unto righteousness, and with the mouth may make confession unto salvation. For also in that psalm, after it had been said, “Who speaketh truth in his heart,” presently this is added, “Who hath used no deceit in his tongue.”