For what knowest thou, O wife, whether thou shalt save thy husband? or how knowest thou, O man, whether thou shalt save thy wife?
By staying with thy unbelieving partner thou mayest save him or her. Enforcing "peace" with the unbelieving consort (1 Corinthians 7:15, end). So Ruth the Moabitess became a convert to her husband's faith: Joseph and Moses probably gained over their wives. Conversely, the unbelieving husband may be won by the believing wife (1 Peter 3:1). Alford explains 1 Corinthians 7:15, 'If thy unbelieving consort wishes to depart, let him go, 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 keep him against his will.'
Save - be the instrument of saving (James 5:20).