Paul deals with this situation in 1 cor 7. It is a little difficult to understand because he says: "I not the lord say". In otherwords, Paul is trying to help people to know what to do, when they find themselves in a situation that is outside of God's will for them. It is not God's will for a unbeliever to be married to a believer. But this is what Paul says to do:
1 Cor. 7:10-16
Now to the married I command, yet not I but the Lord: A wife is not to depart from her husband. [11] But even if she does depart, let her remain unmarried or be reconciled to her husband. And a husband is not to divorce his wife. [12] But to the rest I, not the Lord, say: If any brother has a wife who does not believe, and she is willing to live with him, let him not divorce her. [13] And a woman who has a husband who does not believe, if he is willing to live with her, let her not divorce him. [14] For the unbelieving husband is sanctified by the wife, and the unbelieving wife is sanctified by the husband; otherwise your children would be unclean, but now they are holy. [15] But if the unbeliever departs, let him depart; a brother or a sister is not under bondage in such cases. But God has called us to peace. [16] For how do you know, O wife, whether you will save your husband? Or how do you know, O husband, whether you will save your wife?
Notice Paul says, for the sake of the children, they should stay married. But if they want to depart, then let them depart. We must always put God first and should not let a unbeliever hinder us in any way, even if it means they will depart from us.
Sense one is going to heaven and the other is not (if the unbeliever does not get saved) then they are going to seperate from one another anyways, it is only a question of when. But sense we do not know if the unbeliever will get saved, then Paul leaves it up to the unbeliver if they are going to depart. Thanks, JohnR7