Chapter 4.—A Difficulty as Regards the Chastity of Unbelievers. None But a Believer is Truly a Chaste Man. 2067
What, then, have we to say when conjugal chastity is discovered even in some unbelievers? Must it be said that they sin, in that they make a bad use of a gift of God, in not restoring it to the worship of Him from whom they received it? Or must these endowments, perchance, be not regarded as gifts of God at all, when they are not believers who exercise them; according to the apostles sentiment, when he says, “Whatsoever is not of faith is sin?” 2068 But who would dare to say that a gift of God is sin? For the soul and the body, and all the natural endowments which are implanted in the soul and the body, even in the persons of sinful men, are still gifts of God; for it is God who made them, and not they themselves. When it is said, “Whatsoever is not of faith is sin,” only those p. 265 things are meant which men themselves do. When men, therefore, do without faith those things which seem to appertain to conjugal chastity, they do them either to please men, whether themselves or others, or to avoid incurring such troubles as are incidental to human nature in those things which they corruptly desire, or to pay service to devils. Sins are not really resigned, but some sins are overpowered by other sins. God forbid, then, that a man be truly called chaste who observes connubial fidelity to his wife from any other motive than devotion to the true God.
See Augustins work Against Julianus, iv. 3.264:2068
Rom. xiv. 23.