1 Corinthians 7:12
But to the rest speak I, not the Lord: If any brother hath a wife that believeth not, and she be pleased to dwell with him, let him not put her away.
Note 16 at 1Co 7:12: Paul had just spoken a command from the Lord that married people should not divorce (1Co 7:10), and if they do, they are to remain single or be reconciled to their mates (1Co 7:11). Here Paul added to these instructions with the Lord's permission (1Co 7:6).
In the case where one of the marriage partners is born again and the other is not, Paul said the believer should remain in the marriage if the unbeliever is agreeable to that. However, if the unbeliever wants a divorce, the believing mate does not have to fight to hold the marriage together; the believer can let the unbeliever depart.