Note 5 at 1 Cor. 7:20: Paul's statements in verses 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 1 Cor. 7:12, p. 891). This verse is the real point he is making. Christians should redeem the situations they find themselves in at the time of salvation and not worry about undoing the past.

Just as you cannot unscramble eggs, so you can not rectify mistakes made in marriage before salvation by divorcing the unbeliever. Paul is saying that whatever marital situation a believer finds himself 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 marriage was an adulterous affair from the beginning, then surely God would have them to dissolve that marriage. Paul is 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, meaning "beloved of Jehovah" (2 Sam. 12:24-25). Likewise, a marriage that was totally wrong before salvation can be sanctified (v. 14) and redeemed so that it becomes a blessed relationship.