14 Wherefore, that which is written, “Who speaks 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 profits him nothing, if he hold it not in his heart, that is, if what he speaks, 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 speaks truth in his heart.” Now this truth the catholic as in his heart he speaks, because so he believes, 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 speaks truth in his heart,” presently this is added, “Who has used no deceit in his tongue.”
>15. And as for that saying of the Apostle, “Putting away lying, speak every man truth with his neighbor, for we are members one of another,” far be it that we should so understand it, as though he had permitted to speak a lie with those who are not yet with us members of the body of Christ. But the reason why it is said, is, because each one of us ought to account every man to be that which he wishes him to become, although he be not yet become such; as the Lord showed the alien Samaritan to be neighbor to him unto whom he showed mercy. A neighbor then, and not an alien, is that man to be accounted, with whom our concern is that he remain not an alien; and if on the score of his not being yet made partaker of our Faith and Sacrament, there be some truths that must be concealed from him, yet is that no reason why false things should be told him.
Source: To Consentius: Against Lying (New Advent)