Matthew Henry Commentary 1 Corinthians 10:15-22

"I speak as to wise men; judge ye what I say. The cup of blessing which we bless, is it not a communion of the blood of Christ? The bread which we break, is it not a communion of the body of Christ? seeing that we, who are many, are one bread, one body: for we are all partake of the one bread. Behold Israel after the flesh: have not they that eat the sacrifices communion with the altar? What say I then? that a thing sacrificed to idols is anything, or that an idol is anything? But [I say], that the things which the Gentiles sacrifice, they sacrifice to demons, and not to God: and I would not that ye should have communion with demons. Ye cannot drink the cup of the Lord, and the cup of demons: ye cannot partake of the table of the Lord, and of the table of demons. Or do we provoke the Lord to jealousy? are we stronger than he?" — 1 Corinthians 10:15-22 (ASV)

Did not joining in the Lord's Supper show a profession of faith in Christ crucified, and of adoring gratitude to Him for His salvation? Christians, by this ordinance and the faith professed in it, were united like the grains of wheat in one loaf of bread, or like the members in the human body, since they were all united to Christ and had fellowship with Him and one another.

This is confirmed by the Jewish worship and customs in sacrifice. The apostle applies this to feasting with idolaters. Eating food as part of a heathen sacrifice was worshipping the idol to whom it was offered, and having fellowship or communion with it; just as one who eats the Lord's Supper is considered to partake in the Christian sacrifice, or as those who ate the Jewish sacrifices partook of what was offered on their altar.

It was denying Christianity; for communion with Christ and communion with devils could not be experienced at the same time. If Christians venture into places and join in sacrifices to the lust of the flesh, the lust of the eye, and the pride of life, they will provoke God.