John Calvin Commentary 1 Corinthians 11:20

"When therefore ye assemble yourselves together, it is not possible to eat the Lord`s supper:" — 1 Corinthians 11:20 (ASV)

This is not to eat the Lord’s supper. He now reproves the abuse that had crept in among the Corinthians concerning the Lord’s Supper, specifically their mixing up profane banquets with the sacred and spiritual feast, and doing so with contempt for the poor. Paul says that in this way it is not the Lord’s supper that is partaken of—not that a single abuse entirely nullified the sacred institution of Christ or reduced it to nothing, but that they polluted the sacrament by observing it in a wrong way. For we are accustomed to say, in common conversation, that a thing is not done at all if it is not done correctly. Now this was no trivial abuse, as we will see later.

If you understand the words is not to mean is not allowable, the meaning amounts to the same thing—that the Corinthians were not in a state of preparation for partaking of the Lord’s supper, since they were so divided. However, what I stated a little while ago is simpler—that he condemns that profane admixture, which had nothing in it similar to the Lord’s Supper.