1 Corinthians 7: 12

 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. In the next verse, he gives the same instruction to women, and then tells us that the unbelieving spouse is “sanctified” by the believing one and their children are not “unclean”. This would seem to be an Old Testament metaphor, making a comparison with a Jew being married to a Gentile. I say this realizing that there are several theories out there for understanding Paul’s intentions here. Once again, considering the strange context, I’m loathed to go much further than that, except to remind you that we are in a section dealing with sexual practices. It might just be that Paul is referring to couples in which infidelity has taken place, but there is no way to know for certain. Paul continues in verse 15 to say that if the unbeliever decides to leave the marriage, the believer should let them go in peace, for they are no longer bound together.