John 6 is interpreted in the context of the Lord’s Supper, even though Christ was not at this time giving them instructions for the Lord’s Supper. Furthermore, the Catholic interpretation is identical to the initial and incorrect interpretation of the disciples.
Apparently the “plain reading” of John 6 befuddled nearly every one of the Fathers. Ignatius, Justin Martyr, Irenaeus, Ambrose of Milan, Tertullian, St. Cyprian, John Chrysostom, and too many more to list all believed in the literal presence. Also, it's interesting that John puts this teaching in the midst of the Passover, one year prior to Jesus' own crucifixion (John 6:4), which hurts Luther's argument rehashed here that there's no way St. John is speaking about the Eucharist. However, we should take a look at the text ourselves.
John 6:26-63 will be where I’m quoting from. Jesus repeatedly says “I am the bread of life” “I am the bread come down from Heaven” “I am the living bread”, etc. Then, the Jews protest, “How can this man give us His flesh to eat?” (v. 52). Does Jesus say, “No no, it’s metaphorical.”? No, instead, He continues in His idea and states: “Truly, truly, I say to you, unless you eat the flesh of the Son of Man and drink His blood, you have no life in yourselves.” As if it weren’t enough, He again states: “My flesh is real food, and my blood is real drink.” Pardon me if my hermeneutical gymnastics get in the way here, but I’ll just go ahead and take Jesus’ words at face value. Jesus does not just say that one must eat His flesh, He says that one must gnaw or gnash His flesh (ο τρωγων - the one gnawing/gnashing/eating). Interesting choice of words. In fact, the more people protest, the stronger His language gets!
Jesus' words in John 6:63 should not be seen as a contraction of His previous statements. Jesus did not go through the repeated attempts at explaining that His body was the real bread which comes down from Heaven to contradict Himself. "It is the Spirit who gives life, the flesh profits nothing." Misunderstood, this is shocking. Did Jesus just go through a very lengthy teaching only to turn it on its head to say that flesh profits nothing? No, of course not. For one, as Christians, there better be a hope that Jesus' flesh profits something or else our hope on Calvary is lost (unless you're a modern Docetist). Jesus is discussing belief here. The Spirit is what provides knowledge of the truth (and Truth) (1 Cor. 2:12). Human flesh profits nothing, it does not provide the Divine wisdom and faith needed for this statement (Matthew 16:17). This is made obvious by vs. 64 - "But there are some of you who don't believe." Don't believe what, a metaphor? What's hard about not believing that Jesus is just talking metaphorically?
I quote the second part of his argument here:
We must also remember the accounts of the last supper in the Gospels, which are separate and distinct from this account in John 6. He tells them that the bread was his body, given for them. And then He tells them that the cup was the new covenant in His blood. There is also the command to “do this in remembrance of Me.” It is here that Catholic christology diverges severely from Scripture. The Catholic mass is not a remembrance of Christ, it is an actual re-sacrificing of Christ.
I'll try to be brief. "Do this in remembrance of me" is not "you know, keep me in your mind." The Eucharist was instituted on the backdrop of the Passover. When the Hebrews would celebrate the Passover years after the Exodus, the fathers would say to their families, "This is what the Lord has done for me. He has led me out of Egypt..." etc. Curious. The idea is that the Semitic idea of "remembrance" was not simply a psychological event, but was a reliving or a continual living of that event. Being Jews, the 12 would've understood what Christ here meant.
Also, the Mass is not a re-sacrifice of Christ. The CCC paragraph 1366 states, "The Eucharist is thus a sacrifice because it represents the sacrifice of the cross, because it is its memorial and because it applies its fruit." Represent = makes present. The Mass is the making present of the reality of the sacrifice of Christ upon the cross to the faithful today.
Matthew then quotes from Ephesians 1:22-23 and states:
The body of Christ is comprised of the people who believe in Him, not a piece of bread.
Scripture also calls the body "living stones". Are we "The Thing" from Fantastic four? No...I wish. It's bad form to take a metahpor and make it exclusive of all other meanings.