John 6:51 quotes Jesus as saying, "I am the living bread that came down from heaven. Whoever eats of this bread will live forever; and the bread that I will give for the life of the world is my flesh".
"For the life of the world" is a key phrase here. We understand about human flesh and blood being expended--life being given--in order to sustain other life. It is why we fight wars, ostensibly--so that life can be preserved even at the cost of other lives ending. Ultimately, we understand that wrong that is being done is wrong enough that giving up life to end the wrong is a cost that is worth paying.
Abraham Lincoln put it well in his "Gettysburg Address" when speaking of those who lost their lives--who GAVE their lives--in the Battle of Gettysburg. The reason the people were gathered on November 19, 1863 was to dedicate the cemetery in which were buried those who were killed in that battle: " We have come to dedicate a portion of that field, as a final resting place for those who here gave their lives that that nation might live," Lincoln said.
And yet, those who gave their lives didn't end the work that needed to be done, even as they contributed to it. Lincoln continues:
"It is for us the living, rather, to be dedicated here to the unfinished work which they who fought here have thus far so nobly advanced. It is rather for us to be here dedicated to the great task remaining before us -- that from these honored dead we take increased devotion to that cause for which they gave the last full measure of devotion -- that we here highly resolve that these dead shall not have died in vain."
It is up to those who continue to live to finish the work, to dedicate themselves as those who died dedicated themselves--to be devoted as those who gave "the last full measure of devotion".
This is in essence what Jesus is asking of us in asking us to eat his flesh and drink his blood. This is not a statement of cannibalism. This is a statement saying that we should be as engaged in the work that Jesus was about as Jesus was--that we should enter into the same concern and care for the "life of the world" as he did. That means loving as he loved, healing as he healed, welcoming as he welcomed. He gave "the last full measure of devotion"--killed by those who, unlike him, didn't believe that all of those things should be lived out. And his resurrection vindicates that he had it right, that these indeed are the values and purposes of life in the Kingdom of God. As we share in Jesus' life--as we eat his flesh and drink his blood--we share in his concern for the world--"we take increased devotion to that cause for which JESUS gave the last full measure of devotion"--for the life of the world.
GOSPEL John 6:51-58
51 I am the living bread that came down from heaven. Whoever eats of this bread will live forever; and the bread that I will give for the life of the world is my flesh. 52 The Jews then disputed among themselves, saying, ‘How can this man give us his flesh to eat?’ 53 So 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. 54 Those who eat my flesh and drink my blood have eternal life, and I will raise them up on the last day; 55 for my flesh is true food and my blood is true drink. 56 Those who eat my flesh and drink my blood abide in me, and I in them. 57 Just as the living Father sent me, and I live because of the Father, so whoever eats me will live because of me. 58 This is the bread that came down from heaven, not like that which your ancestors ate, and they died. But the one who eats this bread will live forever.’