1 Corinthians 7:12–13
12 But to the rest I, not the Lord, say: If any brother has a wife who does not believe, and she is willing to live with him, let him not divorce her.
13 And a woman who has a husband who does not believe, if he is willing to live with her, let her not divorce him.
Paul’s exhortation, I, not the Lord, say, carries as much weight as if the Lord had spoken the words. All Scripture is inspired, as are Paul’s words here. Let’s call it his permissible opinion.
If a man became a believer (brother) and his wife was not a believer and is willing to live with him he is not to divorce her (separate from her).
Likewise, Paul’s permissible opinion holds true for the woman believer with an unbelieving husband willing to live with her. Don’t separate.
Paul was addressing Jews in the Corinth church. For a Jew to believe in Jesus as Messiah, it was blasphemous to the unbeliever. There could be and probably were many who would disown family members for their newfound belief, and this would have undoubtedly carried over into the marriage.