53 Jesus said to them, “Amen, amen, I say to you, unless you eat the flesh of the Son of Man and drink his blood, you do not have life within you. 54 Whoever eats my flesh and drinks my blood has eternal life, and I will raise him on the last day.
Most all scholarly works hold that v.53 is at the heart of the matter. In addition to the Protestant/Reformed – Catholic divide, there is a more subtle divide among scholars. Consider the position of Leon Morris  vis-à-vis these verses:
“This is the section of the discourse that is claimed most confidently to refer to the Holy Communion. The language of eating the flesh and drinking the blood is said to be explicable only, or at least most naturally, in terms of the sacrament. But is this so? Surely not! The objections already urged remain, and the very strength of the language is against it. The eating and drinking spoken of are the means of bringing eternal life (v. 54), and they are absolutely unqualified. Who is going to argue seriously that the one thing necessary for eternal life is to receive Holy Communion? Nothing is said, for example, about faith; is it not necessary to believe if we are to have life? Again, “flesh” is not commonly used with reference to the sacrament. In every other New Testament passage referring to it the word is “body.” Ryle further points out that to take the view we are opposing “is to interpose a bodily act between the soul of man and salvation. This is a thing for which there is no precedent in Scripture. The only things without which we cannot be saved are repentance and faith.” I am not contending that we cannot apply the passage helpfully to the sacrament. But I very strongly doubt whether this is the primary meaning. It seems much better to think of the words as meaning first and foremost the appropriation of Christ.”
[Note: In Hebrew and Aramaic of Jesus’ day, there really was no word for “body.” John’s use of “flesh” (whereas the synoptic Eucharistic accounts use “body”) is perhaps closest to the language of Jesus. The earliest writers of the church, e.g. Ignatius and Justin Martyr use the language of “flesh” in their discourses and letters regarding the Eucharist. Clearly the first Christian communities recognized the Eucharistic theme of John’s verses.]
Consider Morris’ interpretive restriction: John’s language cannot stray from the synoptic standard. What Morris seems to take as a given is one of the very challenges facing every reader and scholars: how are we to consider the Gospel of John as regards the synoptic gospels? Morris looks to the synoptic tradition and its sacramental word “body.” He makes a similar move in citing Ryle. These are not arguments to ignore, but does this means that the synoptic and broader NT vocabulary are normative for John? Then again consider O’Day’s  insight: “The interpreter must begin with the miraculous feeding and Jesus’ revelation of himself as the bread of heaven, not with the synoptic Gospels and an imported notion of normative eucharistic theology and practice in the early church. If interpreters of John 6 can free themselves from preconceptions about how a Gospel writer “should” present the eucharist, they will enjoy a fuller understanding of the bread of life discourse and of the eucharist.”
To my mind, this latter point comes across when Morris writes: “Nothing is said, for example, about faith; is it not necessary to believe if we are to have life?… The only things without which we cannot be saved are repentance and faith.” Nothing is said about faith? If you are considering vv.51-58 alone and apart from the rest of John 6 you can perhaps mount an argument. But if one, as O’Day argues, is considering the continuity and integrity of John 6, then faith is the precursor and necessary commitment to this section. Consider 6:29 – “This is the work of God, that you believe in the one he sent” and 6:35 “I am the bread of life; whoever comes to me will never hunger, and whoever believes in me will never thirst.” There seems to be a quite clear path in which coming to/belief in Jesus is being framed as that which satisfies hunger and thirst. But ignoring the continuity and integrity of the whole chapter is the very thing O’Day warns against.
Is anyone surprised that anyone hearing this discourse in person would be naturally perplexed? Up to this point the dialogue has centered on “bread from heaven,” “living bread,” and references aplenty to the manna of the Exodus (and from who the gift came).
51 I am the living bread that came down from heaven; whoever eats this bread will live forever; and the bread that I will give is my flesh for the life of the world.” 52 The Jews quarreled among themselves, saying, “How can this man give us (his) flesh to eat?”
Now Jesus says quite clearly that the bread is his “flesh.” Naturally, one would wonder how this would all be possible. But then this is the same Johannine pattern seen in the chapters 2, 3, and 4. Those present in the Temple who hear Jesus will raise a destroyed temple in three days; Nicodemus who wonders how to be born again/from above; and the Samaritan woman who is initially puzzled by flowing/living waters – some call the pattern misunderstanding. Others would call it a way in which to get people to discern more deeply as to the mystery of God who stands before them. Not all will understand. The Samaritan woman alone sees that Jesus is the promised Messiah. The people in this narrative also face the same doorway. Will they pass through and discover new meaning or will the practical mechanics of “how is this possible” deter them from Truth.
It is hard to know how firmly to hold to the Johannine “misunderstanding” pattern. It is clear if Jesus simply means to use bread and flesh in a metaphorical fashion, then it is lost on the people. Where Nicodemus and the Samaritan woman have choices there are no as-readily apparent choices available to the people. They are confused and one thing is clear: Jesus does not stop and say, “Sorry, let me explain the metaphor.” In fact, he ratchets up language and begins with a quite solemn proclamation – for the fourth time in this Bread of Life Discourse. For Jesus, what follows is paramount.
John 6:53 Amen, amen, I say to you: The presence of the double “amen” in v. 53 makes this the third use of the expression to introduce Jesus’ response to the misunderstanding interruptions that mark the beginning of each section (cf. vv. 26, 32).
John 6:54 those who eat my flesh: The use of trōgein for the action of “eating” is found throughout vv. 53-58 (cf. vv 54, 56, 57, 58). The claim that the verb is used to express the physical experience, “to munch,” “to crunch” is sometimes questioned. Those who reject this physical meaning point to the presence of phagein in the immediate context (cf. v. 53), and thus claim that the verbs are interchangeable. This does not respect the fact that the verbs phagein and esthiein are found in a number of places and contexts in the Fourth Gospel, but trōgein is found only in 6:54-58 and 13:18. Both of these passages have eucharistic background. It is often suggested that the vigor of this language combats emerging docetic ideas about Jesus.
Raymond E. Brown, The Gospel According to John, vol. 29a in The Anchor Bible, eds. William Albright and David Freeman (New York, NY: Doubleday, 1966) 281-94
Leon Morris, The Gospel According to John, The New International Commentary on the New Testament (Grand Rapids, MI: Wm. B. Eerdmans Publishing Co., 1995). 333-37
Gail R. O’Day, John in the New Interpreter’s Bible, Volume 9, ed. Leander E. Keck (Nashville, TN: Abingdon Press, 1996) 607-09
Scripture – Scripture quotes from New American Bible by Confraternity of Christian Doctrine, Inc., Washington, DC. © 1991, 1986, 1970 at http://www.usccb.org/bible/index.cfm