1 Corinthians 7:12-13But to the rest speak I, not the Lord: If any brother hath a wife that believeth not, and she be pleased to dwell with him, let him not put her away. And the woman which hath an husband that believeth not, and if he be pleased to dwell with her, let her not leave him.
1Corinthians 7:15 But if the unbelieving depart, let him depart. A brother or a sister is not under bondage in such cases: but God hath called us to peace.
This is what St Paul reckons should happen between a Christian and a non-Christian. You can marry a non-Christian, but if they want to leave, let them. But as a Christian, you should not leave or divorce your non-Christian spouse.
This is what Jesus says about divorce:
Matthew 19:9 And I say unto you, Whosoever shall put away his wife, except it be for fornication, and shall marry another, committeth adultery: and whoso marrieth her which is put away doth commit adultery.
So it looks like a Christian should not divorce their spouse except for sexual immorality. Jesus didn't, however, say people should divorce an adulterous spouse, only that they can.
Now, supposing both parties are Christian. Ideally, they will both remain faithful, take Jesus' words on board and not commit adultery. However, if one should stray and commit adultery, then it would be easier for the other to forgive and avoid divorce if the adulterous spouse is repentent.