I Corinthians 10:20

But I say, that the things which the Gentiles sacrifice, they sacrifice to devils, and not to God: and I would not that ye should have fellowship with devils. (KJV)

Then: Paul again answers his own questions. This is the first time he actually names the Gentiles' "table partners" (translation by N. T. Wright, p. 130). The Corinthians can no longer escape Paul's reasoning. They already know that in breaking and eating the bread, they share the remembrances of Christ's death - his broken body on the cross, broken for them. Now as they normally celebrate Christ's resurrection through communion, they deny him by eating and celebrating before idols in temples (Baker, p. 148). Even though the idols themselves are not real gods, they represent an evil dimension. Paul wants the Corinthians to understand and flee from all evil. They must to maintain their salvation.

Now: Today's Christians need to understand exactly what happens during communion as much as the Corinthians needed to know. Anything sacrificed elsewhere makes an alliance outside of God's will, offends him, and tempts him to wrath. Have no fellowship with the demons of darkness.

Extra source: Cornerstone Biblical Commentary 1 Corinthians (William Baker), 2 Corinthians (Ralph Martin & Carl Toney), ed. by Philip Comfort. Carol Stream: Tyndale House Publishers, c2006.