Are you taking all of chapter 6 into context when you examine this?
It sure looks like the conversation begins in John 6:22 where the conversation begins regarding food. On more than one occasion Jesus is linking food and belief.
In v27 He tells the crowd to not work for food that will perish but for the food that endures to eternal life.
They respond in v28 by asking how may they work the works of God....His reply in v29 is to believe in Him.
Then they ask for a sign that they may believe and reference God giving their ancestors manna.
Jesus explains the Father has given true bread from Heaven and they reply in v 34 to give them this bread.
In v35 Jesus identifies Himself as the bread of life. He who comes to Him will not hunger and he who believes in Him shall not thirst.
In v36, which is critical to understanding this, Jesus says, "But I said to you, that you have seen Me, and yet do not believe."
He follows up on this in v40 by saying that everyone who believes in Him, may have eternal life.
In v41 the Jews aren't happy as He called himself the bread of life and then attempt to cast further dispersion upon Him by noting: Is this not the son of Joseph, whose father and mother we know? How does He now say I have come down out of Heaven?"
Jesus knows they are grumbling about His statement as noted in v43.
From there He again links Himself with God in vv 44-46 and then restates the need for belief in v47 "Truly, truly, I say to you he who believes has eternal life.
He makes the connection that He is the the bread in 48 that gives life and continues this theme in vv 49-51.
In v52 the Jews debate as to how He can give His flesh for them to eat. They are aware of the OT prohibitions on eating blood so this is blowing their minds.
In vv53-58 we come upon the verses we are debating. These verses relate back to v35 when Jesus notes that those who come to Him will not hunger or thirst. We know this is a metaphor because as Christians we still get hungry and thirsty....but not spiritually. So if this is not literal why do we take the verses in 53-58 as literal when the clear meaning is have faith in Christ...believe in Him.
If we keep all of this in context going back to v22 we have to ask the following:
What message has Jesus been trying to communicate to the Jews?
How has He said we have eternal life?
What has He compared Himself too that came from Heaven?
To reinforce that this is all about believing in Him, He questions His disciples who were grumbling amongst themselves.
In v64 he notes "there are some of you who do not believe."
Peter reinforces this is about faith in vv67-69 when Jesus asks the twelve if they want to leave also.
Peter answers by noting that "You have words of eternal life. And we have believed and have come to know that you are the Holy One of God."
The whole passage is about faith. Do you believe Jesus is the bread of life?
That's what He's asking the Jews and that's the message He still asks today. Is He the bread of life? Do you have faith in Him?
In all of John this is the central question Jesus asks of people....do you believe in Me?
Note that Jesus has just fed the crowd in the desert with miraculous food, like Moses. I.e., Jesus is a prophet, another Moses. If he can feed people miraculously, like Moses, then he can make law like Moses.
This he proceeds to do. He lays it down as law that they must eat his flesh and drink his blood. Yes: this goes counter to the Mosaic Law.
The whole gospel of John is about believing that the Word became flesh.
So focused is John’s gospel on this point that there is little or no moral teaching in the Gospel of John; it is all about whether people believe or disbelieve that Jesus is God made flesh. Each encounter with Jesus ends with a person either worshiping him as God, or plotting to kill him.
The Catholic position on John 6 in no way involves denying any of this.
The Catholic belief in the Real Presence of Christ in the Eucharist is absolutely inseparable from this same act of faith. Obviously, if God did not become Man 2000 years ago, if the Word did not become walking-talking flesh, then it is preposterous to believe that this identical God-made-Man is present with us today through the signs of bread and wine. To believe in the Real Presence in the Eucharist absolutely requires believing that “the Word became flesh, and dwelt among us [2000 years ago].”
Again, the question is, Was Jesus saying something more disturbing to his Jewish audience than recommending to them a MENTAL act of faith? The reaction of the crowd is evidence that he was. St. John never hints that the crowd was revolted at Jesus’ words because they misunderstood Jesus. They repeated his words back to him: How can this man give us his flesh to eat and his blood to drink?
“Give us his flesh to eat and his blood to drink” is NOTHING like saying, “I am like food” or “Food is a good metaphor for me.”