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In marriages with difference of beliefs, the Christian spouse has a particular task: "For the unbelieving husband is consecrated through his wife, and the unbelieving wife is consecrated through her husband."(1 Cor 7:14) It is a great joy for the Christian spouse and for the Church if this "consecration" should lead to the free conversion of the other spouse to the Christian faith.(1 Cor 7:16) Sincere married love, the humble and patient practice of the family virtues, and perseverance in prayer can prepare the non-believing spouse to accept the grace of conversion.
I agree Vincent, Paul was addressing unsaved people who got saved while already in the relationship.
English Standard Version
For how do you know, wife, whether you will save your husband? Or how do you know, husband, whether you will save your wife?