Note 5 at 1 Cor. 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 (1 Cor. 8:10). Paul calls that having fellowship with devils (v. 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 is making the real issue very clear to convince them to abandon this practice.