Translation
For how do you know, oh wife, if you will save the husband? Or how do you know, oh husband, if you will save the wife?
Paraphrase
Can you know for sure, oh wife, whether or not your testimony will result in your husband getting saved? Or can you know for sure, oh husband, whether or not your testimony will result in your wife getting saved?
IT’S UP TO THE SPOUSE
If the unbelieving spouse is willing to stay in the relationship, it may indicate that there is a degree of openness there. If the unbelieving spouse is unwilling to stay in the relationship, that indicates something too. Leave it up to them because they are responsible before God for their own choices; you are not responsible for the choices they make, only your own.