1 Corinthians 7:20
Let every man abide in the same calling wherein he was called.
Note 5 at 1Co 7:20: Paul's statements in 1Co 7:17-24 mention circumcision and slavery, but in fact, they are a continuation of his teaching on the obligation of a Christian mate in an un-Christian marriage (see note 16 at 1Co 7:12). This verse is the real point he was making. Christians should redeem the situations they find themselves in at the time of salvation and not worry about undoing the past.
Just as eggs cannot be unscrambled, so the mistakes made in marriage before salvation cannot be rectified by divorcing the unbeliever. Paul was saying that whatever marital situation believers find themselves in at the time of salvation is where they should stay (this would not apply to extramarital relations or perversions such as homosexuality).
Some think that if their marriages were adulterous affairs from the beginning, then surely God would have them to dissolve those marriages. Paul was speaking against that. It is very clear that King David's relationship with Bathsheba was adulterous. Yet, when he repented, God forgave him and blessed his marriage with Bathsheba to such a degree that the Lord personally named their child Jedidiah (2Sa 12:24-25), meaning "beloved of Jah" (Strong's Concordance). Likewise, marriages that were totally wrong before salvation can be sanctified (1Co 7:14) and redeemed so that they become blessed relationships.