What was the background of the people Paul converted in Corinth?
The people Paul converted in Corinth had all been in idolatry before believing. If they were married, both husband and wife naturally shared that same idolatrous background. This made Corinth's situation unusually severe, unlike churches such as Ephesus, Philippi, and Colossae, where Paul did not have to address the same marital circumstances.
Les explains that Corinth was a special case. He says this is the only letter where Paul deals with these things, so the Ephesians, Philippians, and Colossians were not under this kind of situation, though to a degree yes, but not like Corinth. In Corinth it was beyond comprehension, and every one of Paul's converts had been in idolatry. If they were married, naturally both spouses had been. That idolatrous background lies behind Paul's instructions about a believing spouse and an unbelieving spouse who is steeped in idolatry.