From Catholic Education Resource Center:
The narrative opens on the eastern shore of the Sea of Galilee with the feeding of the five thousand, the only miracle recorded by all four evangelists. After the people were fed, Jesus withdrew to the hillside to be alone. Night fell, and the disciples went down to the lake without Him and, embarking in the only boat available, sailed for Capharnaum, which was on the western shore. Jesus caught up with them some time later by walking on the water. The multitude, thinking He still must be with them, stayed overnight where the miracle was performed. The next morning they discovered Jesus was nowhere to be found and, when other boats put in near them, they embarked for Capharnaum, where they found Jesus and asked Him when (but not how) He had made His way there, apparently thinking He had set off on foot before dawn for the long walk around the lake. He did not answer directly, but told them to “work to earn food which affords, continually, eternal life” (Jn 6:27). He had provided them their fill of natural bread; now He began to speak of supernatural bread.
With verse 30 begins a colloquy that took place in the synagogue at Capharnaum. The Jews asked Him what sign He could perform, and, as a challenge, they noted that “our fathers had manna to eat in the desert” (Jn 6:31). Could Jesus top that? He told them the real bread from heaven comes from the Father. “Give us this bread”, they insisted. “But Jesus told them, ‘It is I who am the bread of life’” (Jn 6:34-35). He was getting more explicit, and the Jews started to complain, but still understood Him to be speaking metaphorically. Jesus repeated what He said before, then summarized: “I myself am the bread that has come down from heaven. If anyone eats of this bread, he shall live forever. And now, what is this bread that I am to give? It is my flesh, given for the life of the world” (Jn 6:51-52). Then the Jews asked, incredulously, “How can this man give us his flesh to eat?” (Jn 6:53).
Hugh Pope, in commenting on this chapter, remarked that **at last “they had understood Him literally and were stupefied; but because they had understood Him correctly, He repeats His words with extraordinary emphasis, so much so that only now does He introduce the statement about drinking His blood”.**1 “You can have no life in yourselves, unless you eat the flesh of the Son of Man, and drink his blood. The man who eats my flesh and drinks my blood enjoys eternal life, and I will raise him up on the last day. My flesh is real food, my blood is real drink. He who eats my flesh, and drinks my blood, lives continually in me, and I in him” (Jn 6:54-57).
This is the only record we have of any of Christ’s followers forsaking him for doctrinal reasons. If they merely had misunderstood Him, if they foolishly had taken a metaphor in a literal sense, why did He not call them back and straighten things out?
There was no attempt to soften what was said, no attempt to correct “misunderstandings”, for there were none. His listeners understood Him quite well. No one any longer thought He was speaking metaphorically. If they had, why no correction? On other occasions, whenever there was confusion, Christ explained what He meant. Here, where any misunderstanding would be catastrophic, there was no effort to correct. Instead, He repeated what He had said.
“There were many of His disciples who said, when they heard it, This is strange talk, who can be expected to listen to it?” (Jn 6:61). These were His disciples, people who already were used to His remarkable ways. He warned them not to think carnally, but spiritually: “Only the spirit gives life; the flesh is of no avail; and the words I have been speaking to you are spirit, and life” (Jn 6:64). But He knew some did not believe, including the one who was to betray Him. (It is here, in the rejection of the Eucharist, that Judas fell away.) “After this, many of His disciples went back to their own ways, and walked no more in His company” (Jn 6:67).
This is the only record we have of any of Christ’s followers forsaking him for doctrinal reasons. If they merely had misunderstood Him, if they foolishly had taken a metaphor in a literal sense, why did He not call them back and straighten things out? Both the Jews, who were suspicious of Him, and His disciples, who had accepted everything up to this point, would have remained had He told them He meant no more than a symbol…
…Leslie Rumble and Charles M. Carty answered this common charge years ago: "There is no logical parallel between the words ‘This is My body’ and ‘I am the vine’ or ‘I am the door.’ For the images of the vine and door can have, of their very nature, a symbolical sense. Christ is like a vine because all the sap of my spiritual life comes from Him. He is like a door since I go to heaven through Him. But a piece of bread is in no way like His flesh. Of its very nature it cannot symbolize the actual body of Christ. And He excludes that Himself by saying, "The bread that I will give is My flesh for the life of the world, and My flesh is meat indeed.’ That is, it is to be actually eaten, not merely commemorated in some symbolical way."6
One last point: In verses 33,35,47,48,50,51,53,55, and 56, Jesus said it is literal what I am telling you. In verses 41,43,52,60, and 64, the Jews doubt, and grumble, and murmur, and in verse 66, they, and many others to this day, walked away from Him and never came back. Did He call them back to say this was only a parable, and explain the meaning of it to them, as he had explained many other parables? No, for in verse 67, He would have let them ALL go, had they all not believed.