10 So far the first section has been explained. Let us now come to those which follow. “But unto the married I give charge, yea not I, but the Lord. That the wife depart not from her husband (but and if she depart, let her remain unmarried, or else be reconciled to her husband): and that the husband leave not his wife. But to the rest say I, not the Lord: If any brother has an unbelieving wife, and she is content to dwell with him, let him not leave her,” and so on to the words “As God has called each, so let him walk.
And so ordain I in all the churches.” This passage has no bearing on our present controversy. For he ordains, according to the mind of the Lord, that excepting the cause of fornication, a wife must not be put away, and that a wife who has been put away, may not, so long as her husband lives, be married to another, or at all events that her duty is to be reconciled to her husband. But in the case of those who are already married at the time of conversion, that is to say, supposing one of the two were a believer, he enjoins that the believer shall not put away the unbeliever.
And after stating his reason, viz., that the unbeliever who is unwilling to leave the believer becomes thereby a candidate for the faith, he commands, on the other hand, that if the unbeliever reject the faithful one on account of the faith of Christ, the believer ought to depart, lest husband or wife be preferred to Christ, in comparison with Whom we must hold even life itself cheap. Yet at the present day many women despising the Apostle's command, are joined to heathen husbands, and prostitute the temples of Christ to idols.
They do not understand that they are part of His body though indeed they are His ribs. The Apostle is lenient to the union of unbelievers, who having (believing) husbands, afterwards come to believe in Christ. He does not extend his indulgence to those women who, although Christians, have been married to heathen husbands. To these he elsewhere says, “Be not unequally yoked with unbelievers: for what fellowship have righteousness and iniquity? Or what communion has light with darkness?
And what concord has Christ with Belial? Or what portion has a believer with an unbeliever? And what agreement has a temple of God with idols? For we are a temple of the living God.” Although I know that crowds of matrons will be furious against me: although I know that just as they have shamelessly despised the Lord, so they will rave at me who am but a flea and the least of Christians: yet I will speak out what I think. I will say what the Apostle has taught me, that they are not on the side of righteousness, but of iniquity: not of light, but of darkness: that they do not belong to Christ, but to Belial: that they are not temples of the living God, but shrines and idols of the dead.
And, if you wish to see more clearly how utterly unlawful it is for a Christian woman to marry a Gentile, consider what the same Apostle says, “A wife is bound for so long time as her husband lives: but if the husband be dead, she is free to be married to whom she will; only in the Lord,” that is, to a Christian. He who allows second and third marriages in the Lord, forbids first marriages with a Gentile. Whence Abraham also makes his servant swear upon his thigh, that is, on Christ, Who was to spring from his seed, that he would not bring an alien-born as a wife for his son Isaac.
And Ezra checked an offense of this kind against God by making his countrymen put away their wives. And the prophet Malachi thus speaks, “Judah has dealt treacherously, and an abomination is committed in Israel and in Jerusalem; for Judah has profaned the holiness of the Lord which he loves, and has married the daughter of a strange god. The Lord will cut off the man that does this, him that teaches and him that learns, out of the tents of Jacob, and him that offers an offering unto the Lord of hosts.” I have said this that they who compare marriage with virginity, may at least know that such marriages as these are on a lower level than digamy and trigamy.
Source: Against Jovinianus (New Advent)