6 I ask my detractors to open their ears and to realize the fact that I have allowed second and third marriages “in the Lord.” If, then, I have not condemned second and third marriages, how can I have proscribed a first? Moreover, in the passage where I interpret the words of the apostle, “Is any man called being circumcised? Let him not become uncircumcised. Is any called in uncircumcision? Let him not be circumcised” (a passage, it is true, which some most careful interpreters of Scripture refer to the circumcision and slavery of the Law), do I not in the clearest terms stand up for the marriage-tie?
My words are these: “'If any man is called in uncircumcision, let him not be circumcised.' You had a wife, the apostle says, when you believed. Do not fancy your faith in Christ to be a reason for parting from her. For 'God has called us in peace.' 'Circumcision is nothing and uncircumcision is nothing but the keeping of the commandments of God.' Neither celibacy nor wedlock is of the slightest use without works, since even faith, the distinguishing mark of Christians, if it have not works, is said to be dead, and on such terms as these the virgins of Vesta or of Juno, who was constant to one husband, might claim to be numbered among the saints.
And a little further on he says: 'Are you called being a servant, care not for it; but, if you may be made free, use it rather;' that is to say, if you have a wife, and are bound to her, and render her her due, and have not power of your own body— or, to speak yet more plainly— if you are the slave of a wife, do not allow this to cause you sorrow, do not sigh over the loss of your virginity. Even if you can find pretexts for parting from her to enjoy the freedom of chastity, do not seek your own welfare at the price of another's ruin. Keep your wife for a little, and do not try too hastily to overcome her reluctance. Wait till she follows your example. If you only have patience, your wife will some day become your sister.”
Source: Letters (New Advent)