In 1 Corinthians 7:12-16, Paul offers this command to Corinthian believers who are married to nonbelievers:
To the rest I say this (I, not the Lord): If any brother has a wife who is not a believer and she is willing to live with him, he must not divorce her. And if a woman has a husband who is not a believer and he is willing to live with her, she must not divorce him. For the unbelieving husband has been sanctified through his wife, and the unbelieving wife has been sanctified through her believing husband. Otherwise your children would be unclean, but as it is, they are holy.
But if the unbeliever leaves, let him do so. A believing man or woman is not bound in such circumstances; God has called us to live in peace. How do you know, wife, whether you will save your husband? Or, how do you know, husband, whether you will save your wife?
Notice several things:
First, the context of these verses indicates that Paul is talking about mixed marriages, i.e., one in which a believer is married to an unbeliever. This differentiates it from verses 10-11, in which Paul spoke directly to Christian couples.
Second, this Paul’s command, not Jesus’. In verses 10-11, Paul appealed specifically to the commandment of Jesus regarding divorce in believing marriages. Jesus did not explicitly address the issue of divorce in mixed marriages, so Paul issues this command on his authority as Jesus’ apostle.
Third, notice that Paul explicitly addresses the believing spouse directly. Though an apostle of Jesus, Paul has no authority over nonbelievers. This commandment applies to believers. And notice that it applies to them whether they are women or men. Indeed, notice throughout this chapter how Paul demonstrates the mutuality of Christian marriage by balancing his remarks to women with similar remarks to men.
Fourth, the most difficult part of this passage has to do with the words “sanctified” and “holy,” which Paul applies to unbelieving spouses and the children of mixed unions, respectively. What Paul is not saying is that unbelieving spouses or children of mixed unions are saved or made holy simply because of their believing parent. This would be contrary to his consistent teaching of salvation by grace through faith (Ephesians 2:8-9).
Rather, the general idea seems to be that by remaining married to an unbeliever, a believing spouse puts his or her unbelieving spouse and children in the position of hearing the gospel and potentially coming to faith themselves. He makes this idea explicit by asking in verse 16 whether a believing wife will “save” her unbelieving husband, and a believing husband his wife.
Finally, Paul allows an exception. A believer may not initiate a divorce simply because of the spouses lack of faith. But if the unbelieving spouse leaves, Paul says this frees them from their marriage vows.
Even in mixed marriages, Paul sees the potential for God to work. If you’re in such a marriage, take heart!
P.S. Check out my review of Peter Hitchens, The Rage Against God: How Atheism Led Me to Faith.