Were the Corinthian believers and their spouses formerly idolaters?
Yes. According to the context, every convert of Paul in Corinth had been in idolatry, and if they were married, then naturally both spouses had been idolaters. The unbelieving spouse is described as still steeped in idolatry, while the believing spouse had come out of that former idolatry.
Les explains that Corinth was a unique situation, the only letter where Paul deals with these things in this way. He says every one of Paul's converts in Corinth had been in idolatry, and if they were married, then naturally both spouses were. The unbelieving husband or wife is spoken of as still being steeped in idolatry, while the believing partner is sanctified in that relationship. If the unbeliever departs, the believer is not under bondage, for God has called us to peace. Yet the believer cannot save the spouse, and the spouse cannot save the believer.