Tertullian, Against Marcion 34

Christ forbids divorce, saying that whoever dismisses his wife and marries another commits adultery, and whoever marries a woman dismissed by her husband is equally an adulterer — so that in this way too he blocks divorce by making the marriage of the repudiated woman illicit. Moses, however, permits repudiation in Deuteronomy: if a man takes a wife and lives with her, and it happens that he finds no favor in her because some indecent matter is found in her, he shall write a certificate of divorce, give it into her hand, and send her from his house (Deuteronomy). You see the difference between the law and the gospel, between Moses and Christ? Obviously. For you have not accepted that gospel of the same truth and the same Christ, in which, while prohibiting divorce, he answers his own question: Moses, because of the hardness of your hearts, commanded you to give a certificate of divorce; but from the beginning it was not so, for the one who made male and female said the two shall become one flesh; what God has joined together, let no one separate (Matthew). By that answer he both safeguarded Moses' statute as his own and upheld the Creator's institution as Christ himself. But to the extent you must be convicted from the texts you have accepted, I will meet you on your own ground, as if my Christ were yours: does he not, while forbidding divorce yet sharing the mind of the one who joined male and female, rather vindicate than demolish Moses' statute? But look — if your Christ teaches the opposite of Moses and the Creator, then he is mine if I do not prove the contrary. For I say he imposed the divorce prohibition conditionally: if a man dismisses his wife in order to marry another, he who dismisses his wife and marries another commits adultery, and he who marries a woman dismissed by her husband is equally an adulterer, on the very ground for which she was dismissed — a ground on which dismissal is not lawful — so that another may be taken; for he who marries a woman illicitly dismissed, as though she were not dismissed, is an adulterer. The marriage remains, since it has not been lawfully dissolved; and while the marriage endures, to marry again is adultery. Thus, if he prohibited dismissing a wife only conditionally, he did not prohibit it entirely, and what he did not wholly prohibit he permitted in other cases where the cause for the prohibition ceases; and so he no longer teaches the opposite of Moses, whose precept he preserves somewhere, not to say confirms. Or if you deny outright that Christ permits divorce, how do you dissolve marriages, neither joining male and female nor admitting those already joined to the sacrament of baptism and the Eucharist, unless they have sworn against the fruit of marriage — that is, against the Creator himself? What will a husband do with you if his wife commits adultery? Keep her? But your own apostle does not allow the members of Christ to be joined to a prostitute. Therefore Christ too upholds the justice of divorce. Now this is confirmed by Moses on the same ground on which Christ also permits repudiation: if an indecent matter is found in a woman. For in Matthew's gospel: whoever dismisses his wife, except for the cause of adultery, makes her an adulteress; and thus the man who marries a woman dismissed by her husband is also counted an adulterer (Matthew).