According to the apostle Paul, the Sunday service in the first century church in Corinth was chaotic. Before we see why, let's be reminded about the origin of this particular church of God. This community came into existence through the ministry of the Apostle Paul. "One night the Lord spoke to Paul in a vision: 'Do not be afraid; keep on speaking, do not be silent. For I am with you, and no one is going to attack and harm you, because I have many people in this city.' So Paul stayed for a year and a half, teaching them the word of God." Through his ministry "many of the Corinthians who heard him believed and were baptised." (See Acts 18:8-11) Though they were God's people, this was a church awash with problems; their carnal behaviour displayed itself when the saints assembled on the first day of the week to partake of the Lord's Supper. What should have been an occasion for holy communion, in every sense of that word, was anything but holy. One of the problems had to do with their previous practice of idolatry.
The converted Corinthians came from a pagan background in which idolatry was the norm. Even after their conversion Paul had to warn them, "Flee from idolatry." He taught them that engaging in idolatry while also partaking of the Lord's Supper was wrong because partaking in the Lord's Supper is "a participation in the blood of Christ... [and] the bread we break [is] a participation in the body of Christ." (1 Corinthians 10:16) Paul provided an explanation of what he meant by way of two illustrations. First, he referred to the religious practice in Israel: when a sacrifice was offered to God a portion of the meat offered was given to the worshipper who ate it and, in this way, he was participating in the proceedings – namely, having fellowship/communion with God. Their eating of the meat linked them to God to whom the sacrifice was offered. Hence Paul's words, "Consider the people of Israel: do not those who eat the sacrifices participate in the altar?" (1 Corinthians 10:18) The answer is yes they do; they have fellowship in the proceedings.
The second illustration was more illuminating to the Corinthians. Paul showed that by offering sacrifices to an idol, the worshipper was having fellowship/communion with demons. He explained that an idol is nothing – it's a piece of carved wood, stone or metal. It is not real. But in another sense it is very real because the idol owes its origin to demons. Paul instructs them that when you offer a sacrifice to an idol, you are "participants with demons." (1 Corinthians 10:21) Paul's point is that demons are present in idolatrous worship; the idol brings the worshipper into contact with the demons. The demons don't enter into the idol, neither do they enter into the sacrifice being offered, but they are nevertheless present. In the same way, God did not enter into the sacrifices Israel offered to him, but nevertheless he was present. The application Paul makes is this: the Lord Jesus Christ is present in the Lord's Supper, not in the sense that he enters into the bread and wine, but he is the unseen guest at the proceedings he instituted. He is the one with whom the partakers of the Lord's Supper are having fellowship. And for that reason Paul reminded these new Christians, "You cannot drink the cup of the Lord and the cup of demons too; you cannot have a part in both the Lord's table and the table of demons." (1 Corinthians 10:21)