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.”
But wasn't it God who fed the people through Moses?
As it was God who provided the manna isn't it God who provides His Son as the bread of life in John 6...as Christ noted in v 32?
Moses did not make the Law. He gave the Israelites what God handed down to him on the mountain.
Neither did Jesus come to change the Law. He said so Himself.
Not one jot or tittle from the Law is to pass away.
The Law was still in effect at the time of the Last Supper. Jesus had not yet completely fulfilled it, so the numerous prohibitions against eating blood were still in effect.