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

For the unbelieving husband is sanctified through his wife, and the unbelieving wife is sanctified through her believing husband; for otherwise your children are unclean, but now they are holy. (1 Corinthians 7:14)

Christians married to unbelievers were not to worry that they themselves, their marriage, or their children would be defiled by the unbelieving spouse. On the contrary, the very opposite was the case. Both the children and the unbelieving spouse would be sanctified through the believing wife or husband.

Being unequally yoked, one flesh with an unbeliever, can be frustrating, discouraging, and even costly. But it need not be defiling because one believer can sanctify a home. In this sense sanctify does not refer to salvation; otherwise the spouse would not be spoken of as unbelieving. It refers to being set apart, the basic meaning of sanctify and holy, terms that are from the same Greek root. The sanctification is matrimonial and familial, not personal or spiritual. In God’s eyes a home is set apart for Himself when the husband, wife, or, by implication, any other family member, is a Christian. Such a home is not Christian in the full sense, but it is immeasurably superior to one that is totally unbelieving. Even if the Christian is ridiculed and persecuted, unbelievers in the family are blessed because of that believer. One Christian in a home graces the entire home. God’s indwelling that believer and all the blessings and graces that flow into the believer’s life from heaven will spill over to enrich all who are near.

In addition, although the believer’s faith cannot suffice for the salvation of anyone but himself, he is often the means of other family members coming to the Lord by the power of his testimony.

A young woman came up to me after the service one Sunday morning and told me that when she was growing up her grandmother was the only Christian in the family. The grandmother used to speak of her love for Christ and witnessed to the family in what she said and by what she did. Eventually, three of the four grandchildren came to know the Lord, and each one declared that their grandmother had the greatest influence on their decision for Christ.

When God was about to destroy Sodom, Abraham pleaded with Him to spare the city if fifty righteous people lived there. “So the Lord said, ‘If I find in Sodom fifty righteous within the city, then I will spare the whole place on their account’ ” (Gen. 18:26). When that many could not be found, the patriarch reduced the number to forty–five, then to forty, thirty, twenty, and finally ten. In each case the Lord agreed to spare the city, but not even ten righteous could be found. But God was willing to bless many wicked people for the sake even of a few of His own people in their midst.

Furthermore, God looks on the family as a unit. Even if it is divided spiritually, and most of its members are unbelieving and immoral, the entire family is graced by a believer among them. Therefore, if an unbelieving spouse is willing to stay, the believer is not to seek a divorce.

The Christian need not fear that the children will be unclean, defiled by the unbelieving father or mother. God promises that the opposite is true. They would otherwise be unclean if both parents were unbelievers. But the Lord guarantees that the presence of just one Christian parent will protect the children. It is not that their salvation is assured but that they are protected from undue spiritual harm and that they will receive spiritual blessing. Because they share in the spiritual benefits of their believing parent, they are holy. Often the testimony of the believing parent in this situation is especially effective, because the children see a clear contrast to the unbelieving parent’s life, and that leads them to salvation.