1 Cor. 10:14-22 (ESV)
Therefore, my beloved, flee from idolatry.  I speak as to sensible people; judge for yourselves what I say.  The cup of blessing that we bless, is it not a participation in the blood of Christ? The bread that we break, is it not a participation in the body of Christ?  Because there is one bread, we who are many are one body, for we all partake of the one bread.  Consider the people of Israel: are not those who eat the sacrifices participants in the altar?  What do I imply then? That food offered to idols is anything, or that an idol is anything?  No, I imply that what pagans sacrifice they offer to demons and not to God. I do not want you to be participants with demons.  You cannot drink the cup of the Lord and the cup of demons. You cannot partake of the table of the Lord and the table of demons.  Shall we provoke the Lord to jealousy? Are we stronger than he?
This passage in 1 Corinthians is absolutely essential to the Lutheran understanding of the sacrament of the Altar. In a moment of extreme lucidity, Paul breaks it down for all Christians of all times. The Cup, it is still wine, but it is the blood of Christ also. The bread, it is still bread, but it is the body of Christ also. Then he compares it to the sacrifices of the Old Testament. You participated in the sacrifices, received their benefit, and confessed your faith, not by burning a goat on the altar, but by eating the sacrifice. Burning the goat didn’t do much of anything, it was the eating that made you a participant in the altar. That is a participant in the feast that God was putting before you. Most sacrifices only had parts of the animal roasting on the altar, those were being consumed by God, who started that fire. The rest of the animal would be eaten by others, the priests, and those offering the animal. In this way you feasted with God, by eating the same animal He was consuming. And in eating with God, He blessed you. It really wasn’t a sacrifice in the sense of giving something up. It wasn’t a chore, it was a feast. So should our sacrament of the altar not be considered a chore to be done every once in a while, but a feast of forgiveness to be celebrated as often as possible.
But here we also see that deaf, dumb, and blind idols have a reality behind them. The food offered in pagan sacrifices are offered to demons, who wear the idols as masks. Non-Christian worship is Satan worship, a rose by any other name…
So I wonder here. We often use this to justify closed communion. I do anyway. I don’t know how far I want to push it though. Most people I deny communion to, are Christian after all. They confess Jesus Christ as their Savior, the Bible as the word of God, etc. (I do not here speak of pseudo Christians, such as JWs, or the LDS who have skewed views of the Trinity that put them outside the realm of Christianity). No it is often others who go by the name Lutheran, or some other reformed denomination, Baptist or Presbyterian. I deny them, at least at first, because I want them to know what they are confessing when they come to the Lord’s Table in our Church. Many would be scandalized by the confession. Most believe it morbid blasphemy to think we eat and drink the body and blood of Christ. That God gives Himself to us under the forms of bread and wine. But that is just it, to deny that it is the body and blood of Christ, is to deny that Jesus was and is God, and makes Him out to be a liar. It also turns the sacrament from being gospel to being law. And any confusion of Law and Gospel that would deny Jesus was and is God, and make Him out to be a liar, must originate with Satan the father of lies. I don’t know that I would say these people worship Satan, but Satan certainly robs them of the comfort of the forgiveness of sins with this blasphemous doctrine, that we do not participate in the altar of the Cross, where Christ sacrificed Himself for the forgiveness of our sins, in the Lord’s Supper.