The Shepherd of Hermas (1st or 2nd Century), Commandment 4, Chapter 1:
“Sir [to the angel], if any one has a wife who trusts in the Lord, and if he detect her in adultery, does the man sin if he continue to live with her?” And he said to me, “As long as he remains ignorant of her sin, the husband commits no transgression in living with her. But if the husband know that his wife has gone astray, and if the woman does not repent, but persists in her fornication, and yet the husband continues to live with her, he also is guilty of her crime, and a sharer in her adultery.” And I said to him, “What then, sir, is the husband to do, if his wife continue in her vicious practices?” And he said, “The husband should put her away, and remain by himself. But if he put his wife away and marry another, he also commits adultery.” And I said to him, “What if the woman put away should repent, and wish to return to her husband: shall she not be taken back by her husband?” And he said to me, “Assuredly. If the husband do not take her back, he sins, and brings a great sin upon himself; for he ought to take back the sinner who has repented. But not frequently. For there is but one repentance to the servants of God. In case, therefore, that the divorced wife may repent, the husband ought not to marry another, when his wife has been put away. In this matter man and woman are to be treated exactly in the same way. Moreover, adultery is committed not only by those who pollute their flesh, but by those who imitate the heathen in their actions. Wherefore if any one persists in such deeds, and repents not, withdraw from him, and cease to live with him, otherwise you are a sharer in his sin.
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Find more of what the early Christians thought on my Christian History page!