1 Corinthians 10:21

Ye cannot drink the cup of the Lord ... - This does not mean that they had no physical ability to do this, or that it was a natural impossibility; for they certainly had power to do it. But it must mean that they could not “consistently” do it. It was not fit, proper, decent. They were solemnly bound to serve and obey Christ, they had devoted themselves to him, and they could not, consistently with these obligations, join in the worship of demons. This is a striking instance in which the word “cannot” is used to denote not natural but moral inability.

And the cup of devils - Demons; 1 Corinthians 10:20. In the feasts in honor of the gods, wine was poured out as a libation, or drank by the worshippers; see Virgil, Aeneas viii. 273. The custom of drinking “toasts” at feasts and celebrations arose from this practice of pouring out wine, or drinking in honor of the pagan gods; and is a practice that still partakes of the nature of paganism. It was one of the abominations of paganism to suppose that their gods would be pleased with the intoxicating drink. Such a pouring out of a libation was usually accompanied with a prayer to the idol god, that he would accept the offering; that he would be propitious; and that he would grant the desire of the worshipper. From that custom the habit of expressing a sentiment, or proposing a toast, uttered in drinking wine, has been derived. The toast or sentiment which now usually accompanies the drinking of a glass in this manner, if it means anything, is now also a “prayer.” But to whom? To the god of wine? To a pagan deity? Can it be supposed that it is a prayer offered to the true God; the God of purity? Has Yahweh directed that prayer should be offered to Him in such a manner? Can it be acceptable to Him? Either the sentiment is unmeaning, or it is a prayer offered to a pagan god, or it is mockery of Yahweh; and in either case it is improper and wicked. And it may as truly be said now of Christians as in the time of Paul. “Ye cannot consistently drink the cup of the Lord at the communion table, and the cup where a prayer is offered to a false god, or to the dead, or to the air; or when, if it means anything, it is a mockery of Jehovah.” Now can a Christian with any more consistency or propriety join in such celebrations, and in such unmeaning or profane libations, than he could go into the temple of an idol, and partake of the idolatrous celebrations there?

And of the table of devils - Demons. It is not needful to the force of this that we should suppose that the word means necessarily evil spirits. They were not God; and to worship them was idolatry. The apostle means that Christians could not consistently join in the worship that was offered to them, or in the feasts celebrated in honor of them.