1 Corinthians 7:13
And the woman which hath an husband that believeth not, and if he be pleased to dwell with her, let her not leave him. And the woman which hath an husband that believeth not, and if he be pleased to dwell with her, let her not leave him.
The instruction concerns a marriage already formed, where the wife has come to faith while her husband remains without belief, and it gives no permission for a Christian to enter marriage with an unbeliever. In that case the believing wife is not to leave her unbelieving husband if he is pleased to dwell with her and willing to live with her. By principle the same duty binds any believing spouse, so the believer must not leave the unbelieving partner who is willing to remain. Conversion does not loosen the bond but strengthens the obligation to remain faithful in marriage even when husband and wife differ greatly in religious faith.
That strengthened faithfulness is to show itself in being faithful, kind and true, and in loving, meek conduct, indeed in even greater faithfulness toward the unbelieving partner. Loyalty in marriage still stands under loyalty to God, so God's claims come first and the believer must hold to conviction rather than surrender what God requires to satisfy unbelief, because Christ is Lord. This includes keeping the Sabbath even when the unbeliever opposes it, and accepting hardship, trial and persecution rather than giving up faithfulness to God. Such steadfast fidelity may exert a sanctifying influence upon the husband and serve to lead him to the truth and help win him.
Opposition, trouble and hardship from the unbeliever do not by themselves justify separation, and the union is to continue unless the unbelieving spouse chooses to depart. It is a serious matter to part a man and his wife, and a believing husband must not put away his wife. But when the wife herself leaves and obtains the divorce and then marries another man, the husband who was left has not broken the marriage bond and is free to marry again, with no scriptural ground for breaking up the new union in order to restore the first. The same principle was applied to counsel that wives with unbelieving husbands or children should not adopt the reform dress when doing so would bring unhappiness outweighing its benefits.