Jamieson, Fausset and Brown on 1 Corinthians 7:16

16. What knowest thou but that by staying with thy unbelieving partner thou mayest save him or her? Enforcing the precept to stay with the unbelieving consort (1Co 7:12-14). So Ruth the Moabitess became a convert to her husband's faith: and Joseph and Moses probably gained over their wives. So conversely the unbelieving husband may be won by the believing wife (1Pe 3:1) [Calvin]. Or else (1Co 7:15), if thy unbelieving consort wishes to depart, let him go, so that thou mayest live "in peace": for thou canst not be sure of converting him, so as to make it obligatory on thee at all costs to stay with him against his will [Menochius and Alford].

save--be the instrument of salvation to (Jas 5:20).