Nicene and Post-Nicene Fathers, Vol. III:
Moral Treatises of St. Augustin: Section 4

4. There is this further, that in that very debt which married persons pay one to another, even if they demand it with somewhat too great intemperance and incontinence, yet they owe faith alike one to another. Unto which faith the Apostle allows so great right, as to call it “power,” saying, “The woman hath not power of her own body, but the man; again in like manner also the man hath not power of his own body, but the woman.” 1943 But the violation of this faith is called adultery, when either by instigation of one’s own lust, or by consent of lust of another, there is sexual intercourse on either side with another against the marriage compact: and thus faith is broken, which, even in things that are of the body, and mean, is a great good of the soul: and therefore it is certain that it ought to be preferred even to the health of the body, wherein even this life of ours is contained. For, although a little chaff in comparison of much gold is almost nothing; yet faith, when it is kept pure in a matter of chaff, as in gold, is not therefore less because it is kept in a lesser matter. But when faith is employed to commit sin, it were strange that we should have to call it faith; however of what kind soever it be, if also the deed be done against it, it is the worse done; save when it is on this account abandoned, that there may be a return unto true and lawful faith, that is, that sin may be amended, by correction of perverseness of the will. As if any, being unable alone to rob a man, should find a partner in his iniquity, and make an agreement with him to do it together, and to divide the spoil; and, after the crime hath been committed, should take off the whole to himself alone. That other grieves and complains that faith hath not been kept with him, but in his very complaint he ought to consider, that he himself rather ought to have kept faith with human society in a good life, and p. 401 not to make unjust spoil of a man, if he feels with how great injustice it hath failed to be kept with himself in a fellowship of sin. Forsooth the former, being faithless in both instances, must assuredly be judged the more wicked. But, if he had been displeased at what they had done ill, and had been on this account unwilling to divide the spoil with his partner in crime, in order that it might be restored to the man, from whom it had been taken, not even a faithless man would call him faithless. Thus a woman, if, having broken her marriage faith, she keep faith with her adulterer, is certainly evil: but, if not even with her adulterer, worse. Further, if she repent her of her sin, and returning to marriage chastity, renounce all adulterous compacts and resolutions, I count it strange if even the adulterer himself will think her one who breaks faith.

Footnotes

1 Cor. 7.41 Cor. vii. 4