The following is an excerpt from The MacArthur New Testament Commentary on 1 Corinthians 7.

Yet if the unbelieving one leaves, let him leave; the brother or the sister is not under bondage in such cases, but God has called us to peace. For how do you know, O wife, whether you will save your husband? Or how do you know, O husband, whether you will save your wife? (7:15–16)

Tertullian (160–230 a.d.), the theologian of Carthage, wrote about heathen husbands being angry with their Christian wives because they wanted to kiss martyrs’ bonds, embrace Christians, and visit the cottages of the poor. Often when an unbelieving spouse wants to leave the marriage the believer has no control over the outcome. But Paul says that Christians should not even try to insist on the spouse’s staying if he or she is determined to go. If the unbelieving one leaves, let him leave. If the unbeliever begins divorce proceedings, the Christian partner is not to contest. Again the word leave (chorizo) refers to divorce.

The brother or the sister is not under bondage in such cases. In God’s sight the bond between a husband and wife is dissolved only by death (Rom. 7:2), adultery (Matt. 19:9), and an unbeliever’s leaving. When the bond, or bondage, is broken in any of those ways, a Christian is free to remarry. Throughout Scripture, whenever legitimate divorce occurs, remarriage is assumed. Where divorce is permitted, remarriage is permitted. It is clearly forbidden in the case in verse 11, but here and in other texts dealing with divorce because of adultery it is not. By implication, the permission given for a widow or widower to remarry (Rom. 7:3; because the person is no longer “joined,” or bound, to the dead partner) can extend to the present case, where a believer is also no longer bound, not under bondage.

God allows divorce in such a case of desertion because He has called us to peace. If the unbelieving husband or wife cannot tolerate the spouse’s faith and desires to be free from the union, it is better that the marriage be dissolved in order to preserve the peace of His child. Fighting, turmoil, bickering, criticism, and frustration disrupt the harmony and peace that God wants His children to have. Again, it is a concession.

“If possible,” Paul says in Romans, “so far as it depends on you, be at peace with all men” (12:18). But when an unbeliever wants out of a marriage, the peace no longer depends on the Christian. Many Christians have tried to keep a marriage together even when the spouse was unbelieving and wanted a divorce. But that course is against God’s will. Let him leave is not permission but a command.

A wife has no assurance that she will save her husband, and a husband has no assurance that he will save his wife. Regardless of a Christian’s motives and hopes, the likelihood of leading the partner to Christ is minimal. If the partner stays in the marriage unwillingly or reluctantly, the likelihood is even less, and the disruption of family peace is assured. The Lord therefore allows no option.

Evangelism is not cause enough to maintain a marriage, especially if the unbelieving partner wants to leave. The believer should let God follow that spouse’s soul with the message of salvation, and use whomever He will to take up the call to faith.