Why did the Corinthian believers have unbelieving spouses?
The Corinthian converts had all come out of idolatry, and if they were married, both spouses originally practiced idolatry. When one partner believed the gospel, the other remained in unbelief, so the believer then had an unbelieving spouse. Paul addresses this situation unique to Corinth.
Les explains that Corinth was a unique case. This is the only letter where Paul deals with these matters, and the Ephesians, Philippians, and Colossians were not under this kind of situation to the same degree. Every one of Paul's converts had been in idolatry, and if they were married, both spouses were idolaters. When one spouse became a believer, the other could remain in idolatry, giving the believer an unbelieving spouse. Paul says the unbelieving spouse is sanctified by the believing spouse, and the children are holy. If the unbeliever departs, let him or her go; the believer is not under bondage, for God has called us to peace. The believer cannot save the spouse, yet Paul asks whether the wife might save her husband or the man his wife.