The first thing to note about the "Eucharistic Discourse" (as it is often called) from John 6 is that it occurs on the eve of the Passover, when the lambs are slaughtered and eaten (cf. John 6:4). This adds greater significance to his words and further points to him as the paschal lamb that must be eaten. After this, he begins to gradually call the Jews to a greater act of faith.
First, Jesus multiplies the loaves and the fishes. This points to the Eucharist in many ways. Jesus will be their nourishment and they will never grow hungry. Also noteworthy is the fact that the account of the miracle begins with almost the same words as those which the synoptics and Paul use to describe the institution of the Eucharist (cf. Mat 26:26; Mark 14:22; Luke 22:19; 1 Cor 11:23-24). This indicates that the miracle is a symbol of the Eucharist, about which our Lord will speak shortly. Upon this miracle, the people begin to believe, but their beleif is imperfect because they see him as an earthly savior and wish to make him king (cf. John 6:14-15).
From this act of faith, he calls for one still greater. Next, he compares himself to the manna which came from heaven. But, while the manna nourished for a time, if they will only believe in him, the true bread, they will be filled forever (cf. John 6:32-35). This is no small claim, to compare oneself to that very bread that saved the Israelites. But this is what Jesus has done........and on top of that, asking that they believe in Him as their spiritual savior. Yet again, the Jews show their lack of perfect faith (cf. John 6:36,41). But, Jesus is not done, for he demands the most sublime act of faith. And what is this sublime act of faith, that will weed out the unbelievers and even cause some of his very disciples to depart from him? It is to eat his flesh and drink his blood.
He begins by saying that this bread from heaven which they will eat is his flesh (cf. John 6:51). The greek word used for "eat" here is favgomai (or "phago"), which means "to eat or consume." In response, the Jews obviously take him literally; they "strove among themselves" (John 6:52). If he was still speaking metaphorically, would this not have been the time to clarify himself? Afterall, he wasn't getting the act of faith he was looking for and it seemed to be because of how they were understanding him. Also, we know that in many similar instances, Jesus explains himself to the people, or at least to the 12 on the side (cf. Mt 16:11-12; Mk 4:34; Lk 12:41-43; Jn 3:3-11) whenever he intends a meaning other than the one they understood.
But, he does not do that here. Instead, he is even more persistent. Starting with vs. 54, we find Jesus telling the crowd 4 more times that they must eat his flesh and 2 more times that they must drink his blood:
54 he who eats my flesh and drinks my blood has eternal life, and I will raise him up at the last day.
55 For my flesh is food indeed, and my blood is drink indeed.
56 He who eats my flesh and drinks my blood abides in me, and I in him.
57 As the living Father sent me, and I live because of the Father, so he who eats me will live because of me.
58 This is the bread which came down from heaven, not such as the fathers ate and died; he who eats this bread will live for ever."
Despite using repitition to drive the point home, he even uses a new, much harsher and more explicit word for "eat." In these instances, the greek word is trwvgw (or "trogo") which means "to gnaw, crunch, or chew." While "phago" may have a spiritual application, "trogo" is never used metaphorically in Greek. It occurs only two other times outside this discourse (cf. Mat 24:38 and John 13:18) and in both cases means a literal eating. It is undeniable what Jesus is asking of them. They must eat his flesh and drink his blood.
Verse 55 from this passage is also quite significant. For one, we are to eat, not his "body" (sw'ma, or "soma"), which often has a metaphorical meaning in the bible, but his flesh. The greek word here is savrx (or "sarx") and it is always used for literal flesh in the bible (proof here). Also, his flesh is meat indeed, his blood drink indeed. The greek word here is ajlhqw'ß (or "alethos"). It means "truly, of a truth, in reality, most certainly" and Jesus uses it to dispel any doubts concerning the reality of His flesh and blood as being food and drink.
In response to this they walk away, including many of his very beloved disciples. But even now, even when he has lost many of his followers, he does not back down. He turns to the faithful remnant, who acknowledge that His is "a hard saying" (John 6:60), and says to them, "Do you take offense at this?" (John 6:61). "Do you also wish to go away?" (John 6:67). He does not intend to retract his statement or to explain it away. Instead, he explains to them why they don't believe it:
61 But Jesus, knowing in himself that his disciples murmured at it, said to them, "Do you take offense at this?
62 Then what if you were to see the Son of man ascending where he was before?
63 It is the spirit that gives life, the flesh is of no avail; the words that I have spoken to you are spirit and life.
64 But there are some of you that do not believe." For Jesus knew from the first who those were that did not believe, and who it was that would betray him
It is often asserted here that, by saying that his words are spirit and life that he is meaning to clarify that he was only speaking symbolically. I find no merit in this claim. For one, nowhere in the bible is the word "spirit" meant to mean "symbol" and nowhere else is something said to be symbolic because it is spiritual. Instead, what we find here is a comparison between the spirit and the flesh that is often used throughout the bible to mean one thing: human wisdom vs. supernatural faith. Both Jesus and Paul use this terminology quite often to point out that we must go beyond the natural to comprehend the supernatural (cf. John 3:6; Mark 14:38; 1 Cor 2:14; 3:3; Rom 8:5; Gal 5:17). Jesus tells us that this is what he meant in vs. 65:
65 And he said, "This is why I told you that no one can come to me unless it is granted him by the Father."
His teaching can only be received by supernatural faith granted by the Father.
In response to all of this, we find the words of Peter in one of my favorite passages from the Bible:
68 Simon Peter answered him, "Lord, to whom shall we go? You have the words of eternal life;
69 and we have believed, and have come to know, that you are the Holy One of God."
Peter too probably had a difficult time with what Jesus was telling them. You can almost feel the desperation in his words. But, he does not abandon Jesus. Instead, he trusts that what Jesus has told them is true, even though it is a mystery beyond full comprehension, one that we can only know by His revelation. Thus, in Peter, Jesus has finally received that most sublime act of faith that he requires from every one of us. I pray for the faith of Peter, and to be counted among the blessed who have not seen and yet believe (Jn 20:29).