“Jesus answered, “The work of God is this: to believe in the one he has sent.”” John 6:29
===
John and Peter, of all the disciples had a special relationship with Jesus. Jesus even allowed them time to commune with Elijah and Moses. John writes in a particular style that is also evident in his letters and in Revelations. Earlier, John had written that Jesus was the Word made flesh. That flesh was sacrificed by God, as a gift to those whom God loved, so that God could abide them Because eternal life is close proximity to God. And to perish is to be utterly riven from God.
Here, Jesus tells his disciples He is the Bread of Life.
The surrounding verse reads
"
When they found him on the other side of the lake, they asked him, “Rabbi, when did you get here?”
Jesus answered, “Very truly I tell you, you are looking for me, not because you saw the signsI 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.”
Then they asked him, “What must we do to do the works God requires?”
Jesus answered, “The work of God is this: to believe in the one he has sent.”
So they asked him, “What sign then will you give that we may see it and believe you? What will you do? Our ancestors ate the manna in the wilderness; as it is written: ‘He gave them bread from heaven to eat.’”
Jesus said to them, “Very truly I tell you, it is not Moses who has given you the bread from heaven, but it is my Father who gives you the true bread from heaven. For the bread of God is the bread that comes down from heaven and gives life to the world.”
“Sir,” they said, “always give us this bread.”
Then Jesus declared, “I am the bread of life. Whoever comes to me will never go hungry, and whoever believes in me will never be thirsty. But as I told you, you have seen me and still you do not believe. All those the Father gives me will come to me, and whoever comes to me I will never drive away. For I have come down from heaven not to do my will but to do the willof him who sent me. And this is the will of him who sent me, that I shall lose none of all those he has given me, but raise them up at the last day. For my Father’s will is that everyone who looks to the Son and believes in him shall have eternal life, and I will raise them up at the last day.”
At this the Jews there began to grumble about him because he said, “I am the bread that came down from heaven.” They said, “Is this not Jesus, the son of Joseph, whose father and mother we know? How can he now say, ‘I came down from heaven’?”
“Stop grumbling among yourselves,” Jesus answered. “No one can come to me unless the Father who sent me draws them, and I will raise them up at the last day. It is written in the Prophets: ‘They will all be taught by God.’ Everyone who has heard the Father and learned from him comes to me. No one has seen the Father except the one who is from God; only he has seen the Father. Very truly I tell you, the one who believes has eternal life. I am the bread of life. Your ancestors ate the manna in the wilderness, yet they died. But here is the bread that comes down from heaven, which anyone may eat and not die. I am the living bread that came down from heaven. Whoever eats this bread will live forever. This bread is my flesh, which I will give for the life of the world.”
Then the Jews began to argue sharply among themselves, “How can this man give us his flesh to eat?”
Jesus said to them, “Very truly I tell you, unless you eat the flesh of the Son of Man and drink his blood, you have no life in you. Whoever eats my flesh and drinks my blood has eternal life, and I will raise them up at the last day. For my flesh is real food and my blood is real drink. Whoever eats my flesh and drinks my blood remains in me, and I in them. Just as the living Father sent me and I live because of the Father, so the one who feeds on me will live because of me. This is the bread that came down from heaven. Your ancestors ate manna and died, but whoever feeds on this bread will live forever.” He said this while teaching in the synagogue in Capernaum."
Read David Daniel Ball's answer to What is the background to, and meaning of John 6:29? on Quora