Note 2 at 1 Cor. 10:18: Paul now uses the Old Testament practice of the priests eating a portion of the sacrifices that were offered on the altar (Lev. 6:18, 29; 7:6, 11-15; 8:31; Num. 18:9-11). In doing so, they were doing more than just eating food. They were illustrating spiritual truths (1 Cor. 9:13-14). No one was allowed to do this but the tribe of Levi.

These Corinthian believers understood and accepted this practice. Yet, if they attached significance to the Jewish priests and their food offerings to the Lord, it was inconsistent to not recognize that there is significance in offering food to idols. This prompted Paul's statement in the next verse that an idol is nothing and food offered to idols is nothing. But there is a communion between those who offer food to idols and the demons behind the idols. Therefore, these believers didn't need to participate.