After reading Fr. Peter Berg's fine treatment of it in the forthcoming issue of Gottesdienst (Christmastide 2012; you can still subscribe to receive this issue HERE), I thought we'd have yet another go at John 6 here and bring some things not covered in Fr. Berg's piece.
It's no secret that John's Gospel is utterly Jewish. It begins with the creation account. It follows the Jewish liturgical calendar (which, incidentally, should give us pause in quoting John 4:23 as reason for worshipping in any way we so choose). Thus, John's Gospel demonstrates that our Lord is the fulfillment of every major feast. He is the Passover Lamb of God, the Bread of Heaven, the Light of the World, the Word of the Lord--the perpetual ordinance and everlasting covenant--the tabernacle and the temple of God in the flesh.
So why is it that we somehow forget this when we get to John 6? Why is it that when our Lord says His flesh is true meat and His blood true drink, we suddenly get skiddish and talk in abstraction as if our Lord was telling us to do something else, something different? John's Gospel is concrete and that is why he uses signs. The sign doesn't point beyond itself. The sign carries the message. The sign is the message. This is how we are to read the whole of John's Gospel.
Jesus is the Lamb of God. He is the Passover sacrifice. He is the Bread of Life come down from heaven. His flesh is meat. And it gives life to those who eat it at His Word.
The Passover lamb was to be roasted and eaten. The Passover bread was to be eaten (Exod 12). The manna and quail were to be eaten (Exod 16). The flesh (BHS: בָּשָֽׂר) of the sacrifice were to be eaten by the Levites, the priests, as a provision of holy food for them from the Lord's table (Lev 6-7). The lamb and the quail were the flesh (BHS: בָּשָֽׂר) that the Lord provided to His people to give them life: Life from out of death in the idolatrous slavery to the Egyptian gods and life out of death as they wandered the wilderness. They ate flesh (BHS: בָּשָֽׂר). They ate it at the Lord's command with the Lord's promise. It was the flesh of the sacrifice (Exod 12; 16; Deut 12:20-28; Deut 16). But it was dead flesh (LXX: κρέα, which always refers to dead flesh, flesh that has no life in it). They could eat the flesh (LXX: κρέα), but they could not eat living flesh (σὰρξ), flesh with the blood in it because the life was in the blood (Deut 12:23). And so they ate the flesh provided by the Lord, they ate the Passover lamb, the manna and quail, the holy flesh of the sacrifices to make them holy. They ate dead flesh and they died.
But the flesh that Our Lord Jesus gives us is no dead flesh (κρέα). The flesh He gives is His living flesh (σὰρξ, which has a wider range of meaning that includes both living and dead flesh like that of the Hebrew בָּשָֽׂר). The flesh He gives is His living, life-giving, risen flesh. It's the same flesh that He took on when the Word, the everlasting covenant and perpetual ordinance of the Father, became flesh to tabernacle among us (John 1:14). And He gives it to us with the promise that it will give life, and not just life to live another day, another week, or another year, but life eternal (John 6:53-58).
So in John 6, Jesus tells us that His living, life-giving, risen flesh is to be eaten. He tells us that His sacrificial flesh offered upon the cross and roasted in the Father's wrath against sin is to be eaten to give us life. Where is this eaten but in the Lord's Supper? Where do we eat at the Lord's table, the holy food that He gives to make us holy and give us life but in the Lord's Supper? Where do the holy priests of the Lord eat the flesh of the sacrifice but in the Lord's Supper?