Note 5 at 1Co 10:21: These Corinthian believers were actually entering into idol temples and eating the food that had been sacrificed there, in the name of spiritual liberty (1Co 8:10). Paul called that having fellowship with devils (1Co 10:20).

No true believer would argue that we should serve the Lord and the devil at the same time. These believers were deceived. They weren't seeing the situation for what it was. Paul was making the real issue very clear to convince them to abandon this practice.