Every Protestant commentary that I have read concerning this passage interprets Jesus' words as symbolic and not literal. Roman Catholics tend to see here their doctrine of Transubstantiation in Communion (bread & wine turn into body and blood of Christ at the moment of ingestion). But Jesus' folowers and the Twelve seem offended not by the symbolism nor by some new mystical practice of communion but by a literal understanding of Jesus' words. We know this by the question they pose to each other: "How can this man give us his flesh to eat?” (John 6:52) and "This is a hard teaching. Who can accept it?" (John 6:60). 

Clearly the eating of Jesus' flesh and drinking His blood was related to faith in Him, after all, He also told the people that the work that God required was "to believe in the One He has sent" (John 6:29). But the people pressed Him for a definitive sign, like the manna given to the Israelites in the wilderness. To these people that manna was proof positive that God was God. So Jesus confronted their request for a manna-like sign by saying that He was the manna from heaven. Just like the Israelites ate the manna, they should now eat His flesh and drink His blood. At this, they thought he was out of His mind.

Now, Jesus seemed to clarify by saying, "The words I have spoken to you—they are full of the Spirit and life," but it is unclear whether the people truly understood that He was only speaking metaphorically, and this statement also does not make that clear. Regardless, we are told many of his disciples stopped following Him (John 6:66). 

Now, ultimately we know that these disciples left because of their unbelief (John 6:64), but why would Jesus take this rather confusing and difficult approach?

According to Deuteronomy 8:3 and16, we learn that God gave the people of Israel manna in the wilderness, a food that was foreign and soon tiresome to them, not so much to prove Himself, but to test them. He also made them hunger, and He humbled them. All this was to teach them that "man does not live on bread alone but on every word that comes from the mouth of God." This was the test: Would they trust in Him in spite of their hunger? Would they follow Him in spite of this foreign and tiresome manna? Would they obey His Word simply because it was God who was speaking to them and not because of His benefits? This was the test, and, in large part, they failed. 

Now, coming back to Jesus and the crowds, who just experienced a profound miracle, where Jesus fed five thousand of them with only few loaves and fish. After this, they were amazed and followed Him. But Jesus declared about them:

Very truly I tell you, you are looking for me, not because you saw the signs I performed but because you ate the loaves and had your fill. Do not work for food that spoils, but for food that endures to eternal life, which the Son of Man will give you. For on him God the Father has placed his seal of approval" (John 6:26-27).

They were trusting the miracle and not Jesus. Their faith only went so far as their bellies were filled. Even after Jesus explains that what God required is faith in Him (John 6:29), they still clamored for more miraculous bread (John 6:34). 

God gave Israel manna in the wilderness to reveal the unbelief in their hearts. Their complaints and threats to return to Egypt revealed that they did not truly trust in Him. If they had truly trusted in God, they would have obeyed Him in spite of the hardship. Their grumbling and complaining only revealed that they were following God so long as He met their immediate needs. 

Jesus spoke of eating His flesh and drinking His blood, "a hard saying," in order to test His followers. Their grumbling among themselves revealed that they did not truly believe in Him. As Jesus would also confirm: "But as I told you, you have seen me and still you do not believe" (John 6:36). Just as God tested Israel by straining their hunger pangs, Jesus tested His followers by straining their sensibilities. The correct response was given by Peter, representing the Twelve:

Lord, to whom shall we go? You have the words of eternal life. We have come to believe and to know that you are the Holy One of God.” (John 6:68-69). 

If Jesus is the Son of God, what did it matter that He commanded them to eat His flesh and drink His blood. The crowds should have gotten out some salt and pepper. They should have offered up their empty cups. The correct response was to simply believe. The issue was not whether what Jesus said seemed sensible to them but Who it was that was speaking to them. He was "the Holy One of God," who had "the words of eternal life." 

Jesus did not clarify the cannibalism His listeners thought He was requiring because He wanted them to believe in Him in spite of what appeared to be a difficult request. It was a test to reveal their unbelief, that they did not follow Him for Who He was but for the benefits that they could receive from Him. And when it appeared that those benefits were no longer forthcoming or suspect, they left. Their faith was fickle, just as the faith of those that first saw His miracles and "believed"in Him.

Now while he was in Jerusalem at the Passover Festival, many people saw the signs he was performing and believed in his name. But Jesus would not entrust himself to them, for he knew all people. He did not need any testimony about mankind, for he knew what was in each person (John 1:23-25).