Q. Why did Jesus say, “Unless you eat the flesh of the Son of Man and drink his blood, you have no life in you”? Whatever he meant was so hard to understand that some of his own followers left when he said this. What’s this all about?
This statement by Jesus needs to be understood in light of two important distinctives of the gospel of John.
First, as I explain in my study guide to that book, “The festivals and locations that Jesus visits allow his identity to be disclosed against the symbolic background of Jewish religious life and history.” In this case, when Jesus journeys across the Sea of Galilee and back at the time of Passover, “The focus is on the event that Passover commemorates: the exodus of the people of Israel from Egypt under the leadership of Moses. . . . While Jesus is on the far shore of the lake, he miraculously feeds a large crowd. When the crowd returns to the opposite shore, they compare this feeding with the manna, the ‘bread from heaven,’ that Moses gave the Israelites in the wilderness.”
Next, as I also explain in my guide, in this gospel Jesus has “conversations . . . with many different people,” and these conversations “tend to follow a certain pattern. Jesus speaks of spiritual realities, but his listeners misunderstand him and think he’s speaking about material realities. They ask questions to try to clear up the confusion, and this gives Jesus (or John, speaking as the narrator) the opportunity to explain the spiritual realities further,” often in an extended discourse.
Jesus’ discourse after the miraculous feeding is designed to explain its meaning. “Jesus turns the crowd’s focus from the sign itself to what it reveals about who he is. He wants them to see him not as the one who gave the bread, but as the one who is the bread. His identification of himself with the manna, the ‘bread from heaven,’ points to his heavenly origins and the divine life he imparts.”
And so Jesus explains in his discourse, “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.”
In other words, when Jesus says “I am the bread of life” and then refers to “eating this bread” in order to have life, what he’s actually talking about is people “believing” in him. As he says in this same discourse, “Everyone who looks to the Son and believes in him shall have eternal life.”
Unfortunately, the crowds misunderstand Jesus and think he is talking about material realities (food and drink, or even his own flesh and blood). Some of them are so confused and scandalized that, as John reports, they “turned back and no longer followed him.” But when Jesus asked the Twelve who were closest to him whether they wanted to leave too, Peter, speaking for all of them, replied, “Lord, to whom shall we go? You have the words of eternal life. We have come to believe and to know that you are the Holy One of God.”
This is the response that John wants all readers of his gospel to make as well, by seeing through the material elements that are literally under discussion to the spiritual realities behind them.
A footnote to this discussion: As I also note in my study guide, “Many interpreters believe that Jesus’ words here about ‘eating his flesh’ and ‘drinking his blood’ are a reference to the Lord’s Supper or Eucharist. These interpreters point out that John doesn’t describe anywhere else in his gospel how Jesus instituted this sacrament. They suggest that John may therefore be doing that here. Eucharistic themes do run through the gospel. For example, the two things that Jesus provides miraculously are wine (at the wedding in Cana) and bread (on the far shore of the Sea of Galilee).” However, if Jesus’ statement is in some way a reference to the Eucharist, the intention is clearly not for people to see eating the material elements of bread and wine as the way to “have life.” Rather, this act is properly an expression of a person’s belief in Jesus. That is the spiritual reality behind this physical and sacramental act.
Good Question 
You may find it interesting to learn that Jesus is recorded as using the word “covenant” only once in all of the New Testament. This one time is over the bread and wine, as he said, this is my Body and Blood. It is the New Covenant! You might find a pretty good explanation of the Upper Room as you watch “The4thcup.com”. It’s a little amateurish, but worth watching. Thanks for your work in Christ.
Christ never said ‘drink, this is my blood’. He said, “for this is my blood of the covenant, which is poured out for many for the forgiveness of sins.” blood of the convenant is one phrase meaning, you no longer live under the old covenant…the law…but you now live under a new covenant which also required blood. Christ never would have let them drink real blood as that was also a law…not drinking life blood. so the disciples knew what he was saying. and He would further explain through the book of Hebrews that He would die ONCE for the many. not be sacrificed over and over every sunday through communion. communion was a reminder of what our work is…to come to Him (eating of flesh) and to believe in Him (drinking the blood). the flesh and blood were His sacrifice on the cross. so you could even boil this down further to say that the work of Christians is to come to the cross where the price of our sin was paid once and for all.