From The Gospel of John, by Francis Martin and William Wright, commenting on John 6:41-51
Jesus spoke of “food that endures for eternal life” (6:27) and then was challenged by the crowd to perform a sign greater than the manna (6:30–31). Jesus went on to speak of “the bread of God . . . which comes down from heaven” (6:33) and identified himself as this heavenly “bread of life” (6:35).
Jesus now returns to the manna, and his discourse crescendos in the revelation that the bread from heaven that gives eternal life is the crucified and glorified flesh of Jesus himself (6:48–51). With strong realism, Jesus teaches that he gives this very same flesh as nourishment to believers in the Eucharist, the sacrament of his body and blood (6:53–58).
Returning to the themes of manna and life-giving bread, Jesus compares himself as the bread of life with the manna. The manna was a providential gift from the Lord to sustain the Israelites in the wilderness. But despite its wondrous nature, the manna did not give eternal life: Your ancestors ate the manna in the desert, but they died. However, the bread of life from heaven does what the manna could not: it gives eternal life, so that whoever eats this bread will live forever.
The food that gives immortality is an allusion to the tree of life in the garden of Eden. According to Genesis, the tree of life’s fruit could give immortality. After their sin, God expelled Adam and Eve from Eden to keep them from eating this food (Gen 3:22–23). Now Jesus says that anyone who eats the bread he gives will live forever. Jesus opens the way to paradise and offers the food that gives immortality.
Then Jesus explains what it means to eat the living bread that came down from heaven. At one level, in light of the biblical imagery for God’s wisdom and Torah, eating this bread means taking Jesus in as spiritual nourishment and wisdom. But there is a much greater depth to his words. Jesus now specifies that this bread that gives eternal life is his own flesh. He gives his flesh for the life of the world in his perfect act of love and obedience on the cross. Once crucified and transformed by the resurrection, Jesus’ human flesh becomes the source of eternal life for the whole world. It is Jesus’ own flesh that people must eat.