1 Corinthians 7:14
For the unbelieving husband is sanctified by the wife, and the unbelieving wife is sanctified by the husband: else were your children unclean; but now are they holy.
Note 17 at 1Co 7:14: One of the probable reasons that the Corinthians desired clarification on this matter was because many of them were in mixed marriages. Their mates not only were non-Christians, but also may have been involved in worshiping other gods and in pagan sexual orgies (see note 1 at Ac 18:1). They questioned whether or not God wanted them united with unbelievers through marriage. Wouldn't that defile them? How would such marriages affect their children? This verse answers these questions.
The power of the Lord in the Christian is stronger than the power of the devil in the unbeliever in the marriage union. The believer sanctifies (sets apart) the unbelieving mate from the defilement that he or she would otherwise bring into the marriage. The Lord purifies the marriage situation through the believing mate so that even the children are blessed instead of cursed.
Like any of the other benefits of salvation, this sanctifying power of the Christian mate has to be released through faith. It is not automatic, but it is available to every believer so that any marital situation can be sanctified.