Here we see Christ miraculously multiplying five loaves and two fishes to feed the multitude that had been following to hear him. After this event many continued to follow him not because they had seen the signs He worked but because they wanted a king who would feed them earthly food. Christ now begins to talk to them about the true bread of life. He said to them,
"I am the living bread
which has come down from heaven;
whoever eat this bread will life forever."
The people were puzzled, we know he is the son of Joseph and Mary, how could he have come down from heaven? Why should we believe Him, let him prove it to us by a great sign. After all during the journey in the desert God had given them manna, "bread from heaven".
Jesus points out the difference of this bread. Look, Jesus said to them, your ancestors ate the manna in the desert and they died but the bread that I will give will make you live forever. So they said to Jesus to always give them this bread. Then came the final test of faith, when Jesus told them,
"Unless you eat
the flesh of the Son of Man
and drink his blood,
you do not have life within you."
I Am the Bread of Life
The Jews were familiar with eating sacrificed meat during the Passover, but they thought, must we now eat the flesh of human being? This is impossible to accept. Jesus did not give any further clarification because they either accepted Him on faith as the Son of God or they did not. The disciples who left wanted to understand with their minds leaving no room for mystery. They did not have the humility to say "I believe" even if I don't understand.
Then Jesus turned to the twelve and said,"'Do you also want to leave?'" Simon Peter speaking for all of them said that they had no one else to follow who could teach them the way of everlasting life except Jesus whom they believed was the Holy One of God.
I Am the Vine