2. The metaphor of flesh/blood are 'mixed', the standard tip-off to metaphor (e.g., "I am the living bread", "I am bread which descended from heaven", bread is actually 'flesh', eating/drinking somehow don't consume Jesus since that is the condition for 'abiding' in Him and since He would still be around at the 'last day').
3. The "participation" theme is quite explicit, especially since it is paralleled to Jesus' dependence on the Father (vs.57). The "as...so" parallel construction there highlights this point: Just as Christ draws His life from the Father through participation in His life, so too the follower of Christ is to draw life from Jesus. The first term of the comparison (i.e. Jesus drawing life from the Father) makes the second term (i.e. drawing life from Christ because of absorption of His life and death) obvious.
4. The objections of the grumblers in the passage have nothing to do with the offensiveness of cannibalism; they get lost in either the logistics (e.g., how can a living person share His flesh and still continue to live?) or in the demands for total dependence on Christ for eternal life ("you have no life, if you don't have me"). If they were understanding this rather completely literally (as opposed to some "moving metaphor complex" of "bread/flesh/life/object of trust") their objections and responses would have been markedly different. In fact, they were arguing among themselves about his meaning, indicating that it was certainly not an obvious reference to cannibalism. Carson has an interesting image of this (his comm. on John, Eerdmans):
"The Jews began to argue sharply among themselves. The very (emachonto) is very strong. Any dullard could see that Jesus was not speaking literally: no-one would suppose Jesus was seriously advocating cannibalism and offering himself as the first meal. But if his language was figurative, what did he mean? Perhaps one argued for this view, another for that, all of them repeating the same literal, unintelligent question to get at the point: How can this man give us his flesh to eat?"
5. It is interesting to note that Jesus expands the metaphor of 'bread/flesh' to include the 'blood' (symbol of His violent and sacrificial death) in vs.53. This would highlight the necessity of participating in His death--not just in His victorious future life.
6. But there may be something stronger is this reference to "drinking of blood"--perhaps a reference to violence against Jesus.
If we look at references to "drinking of blood" (and eating of flesh) in the OT, a number of passages make reference to this (without, by the way, Cannabalistic or ritualistic overtones):
Behold, a people rises like a lioness, And as a lion it lifts itself; It shall not lie down until it devours the prey, And drinks the blood of the slain.” [numbers 23.24, in which Israel, under the figure of a lion, will "drink blood"]
And the sword will devour and be satiated And drink its fill of their blood; [Jer 46.10]
This might indicate further that Jesus is alluding to His coming rejection and death. [The passage in 1 Chr 11.19: "nevertheless David would not drink it, but poured it out to the Lord; 19 and he said, “Be it far from me before my God that I should do this. Shall I drink the blood of these men who went at the risk of their lives? For at the risk of their lives they brought it.” Therefore he would not drink it. ", shows the link between accepting responsibility for someone's death (or risk of death) and "drinking blood"--it was a way of saying "I am responsible for this death", not "I am a cannibal"...]
Jesus' usage of this metaphor here is quite in keeping with Jewish and proto-rabbinic usage of the day, and not at all sacrilegious:
1. Rabbinic literature would use the 'eating and drinking' metaphor for absorption in Torah and good works. The midrash on Ecc. 2:24 says specifically that "All references to eating and drinking in the book of Qohelet signify Torah and good works." Jesus' use of 'eating and drinking' herein would have been in good rabbinic style.
2. . Bread is also very frequently used metaphorically in the rabbinics for 'doctrine'. For example:
"It is written: For, behold, the Lord, the Lord of hosts, doth take away from Jerusalem and from Judah stay and staff every stay of bread, and every stay of water,’ the mighty man, and the man of war; the judge and the prophet, and the diviner, and the elder; the captain of fifty; and the man of rank, and the counsellor, and the wise charmer, and the skillful enchanter. And I will give children to be their princes, and babes shall rule over them. 27 ‘Stay’ — this means the masters of the Bible...Every stay of bread’ — this means the masters of Talmud, 31 for it is said: Come, eat of My bread, and drink of the wine which I have mingled" (Chagigah 14a)3. Even Josephus could refer metaphorically to 'drinking of blood' without it being a problem:
For hitherto they [warriors of the Jewish resistance before the fall of Jerusalem] had fed themselves out of the public miseries, and drank the blood of the city. [Wars of the Jews, 5:344]
4. And, to be even more clear, the rabbi's even spoke of 'eating the Messiah' when he appeared (and without any cannibalistic overtones or objections), and by that meant a sharing and enjoying of His benefits--exactly what Jesus is referring to here:
"R. Giddal said in Rab's name: The Jews are destined to eat [their fill] in the days of the Messiah.[ lit. "Israel shall eat the years of Messiah"] R. Joseph demurred: is this not obvious; who else then should eat — Hilek and Bilek? — This was said in opposition to R. Hillel, who maintained that there will be no Messiah for Israel, since they have already enjoyed him [literally, "devoured him"] during the reign of Hezekiah. [Sanh. 98b, Hillel's words repeated in 99a]
Again, as we saw in the Last Supper, the idea is that of intimacy with the Messiah--the closest possible alignment and identification with His life and scandalous death. The images and metaphors used were part of the rabbinic 'stock' of ideas by which to express messianic and salvific hopes (although some of the way Jesus used them seemed to be aimed at 'shocking' them into listening to what he was saying--instead of operating on their own messianic models.). One main difference, however, is that the intimacy/participation is with His comphrensive mission (including His humiliation--cf. 'take up your cross and follow Me'), not just the politically 'victorious' one.