In 1Cor 11:17-19, Paul criticizes the Corinthians for acting like high schoolers by forming cliques, eating too much, and getting drunk.
- That some people were eating too much is implied when he mentions that "one is hungry". Ever been last in line at a potluck only to find that all the food is gone?
- There probably weren't any 'food fights', or outright 'fighting' over food, but they were likely cutting in line ("taketh before other his own supper"). It isn't too different from how some people behave today.
So then, when you come together, it is not the Lord’s Supper you eat, – 1Cor 11:20 (NIV)
The phrase "so then" refers back to his earlier criticism. It's implied that they're supposed to be eating the Lord's Supper, but they aren't because of their behavior.
Many religions conclude worship with a substantial meal. Sacrificial animals were cooked (as burnt offerings) and served in meals, which Paul describes in 1Cor 8 and 10. It's likely the early church followed similar practices, and the Lord's Supper did consist of a real meal. Although no longer considered part of the Lord's Supper, some modern churches do have potlucks.
Paul describes the ritual in verses 23-26 to remind the Corinthians that their behavior is antithetical to remembering Lord Jesus. Not to tell them to replace the meal with a wafer and sip of wine.
So then, whoever eats the bread or drinks the cup of the Lord in an unworthy manner will be guilty of sinning against the body and blood of the Lord. – 1Cor 11:27 (NIV)
He ends by telling them to get their act together. Stop falling asleep in church, and be more disciplined. (28-32) And eat together as a church, not in your own little groups (that's why it's called communion). (33) And eat breakfast before coming to church, so you won't cut in line, causing others to "judge" (34) (because you already know the sermon is going to be long – that's why you fell asleep while I was still talking).