What does 1 Corinthians 10:21 mean?

Sacrifices offered to false idols are, even unknowingly, offered to demons. Paul's reason for not wanting the Christians in Corinth to have anything to do with idols or idol food has become clear. To participate with idols is to participate with demons. Paul's prior remarks clarified that there is no overt sin in eating such meat (1 Corinthians 8:4–7), and he will say as much later (1 Corinthians 10:25). The problem is not in the physical meat, but in the attitude of the person who eats it.

Now Paul insists that participation with demons is mutually exclusive with participation with Christ. The same person cannot both drink the cup of the Lord and the cup of demons. To drink from another person's cup, in this context, implies joining that person in his or her destiny (Matthew 20:22–23). Also, Christians drink from Christ's cup when they participate in communion.

To knowingly participate in idol worship on any level involves that person in drinking from the cup of a demon. The destiny of demons is eternal destruction. The destiny of Christ is eternal glory. To participate in both is unthinkable for Paul.

Then he gets more specific. Because this is true, the same person should not participate in the table of the Lord, through communion, and in the table of demons, through the eating of food sacrificed to idols. It's not that the food, in either case, necessarily carries some supernatural power. It's that the act of eating from those tables is an act of joining oneself to that specific "lord."

Nobody can continue to worship both Christ and a demon. A choice must be made.