From: John 6:30-35
The Discourse on the Bread of Life (Continuation)
 So they said to Him (Jesus), "Then what sign do You do, that we may see, and believe You? What work do You perform?  Our fathers ate the manna in the wilderness; as it is written, `He gave them bread from Heaven to eat.'"
 Jesus then said to them, "Truly, truly, I say to you, it was not Moses who gave you the bread from Heaven; My Father gives you the true bread from Heaven.  For the bread of God is that which comes down from Heaven, and gives life to the world."  They said to Him, "Lord, give us this bread always."
 Jesus said them, "I am the bread of life; he who comes to Me shall not hunger, and he who believes in Me shall never thirst."
28-34. This dialogue between Jesus and His hearers is reminiscent of the episode of the Samaritan woman (cf. John 4:11-15). On that occasion Jesus was speaking about water springing up to eternal life; here, He speaks of bread coming down from Heaven to give to the world. There, the woman was asking Jesus if He was greater than Jacob; here the people want to know if He can compare with Moses (cf. Exodus 16:13). "The Lord spoke of Himself in a way that made Him seem superior to Moses, for Moses never dared to say that he would give food which would never perish but would endure to eternal life. Jesus promises much more than Moses. Moses promised a kingdom, and a land flowing with milk and honey, good health and other temporal blessings [...], plenty for the belly, but food which perishes; whereas Christ promised food which never perishes but which endures forever" (St. Augustine, "In Ioann. Evang.", 25:12).
These people know that the manna--food which the Jews collected every day during the journey through the wilderness (cf. Exodus 16:13ff)-- symbolized messianic blessings; which was why they asked our Lord for a dramatic sign like the manna. But there was no way they could suspect that the manna was a figure of a great supernatural messianic gift which Christ was bringing to mankind--the Blessed Eucharist. In this dialogue and in the first part of the discourse (verses 35-47), the main thing Jesus is trying to do is bring them to make an act of faith in Him, so that He can then openly reveal to them the mystery of the Blessed Eucharist--that He is the bread "which comes down from Heaven, and gives life to the world" (verse 33). Also, St. Paul explains that the manna and the other marvels which happened in the wilderness were a clear prefiguring of Jesus Christ (cf. 1 Corinthians 10:3-4).
The disbelieving attitude of these Jews prevented them from accepting what Jesus revealed. To accept the mystery of the Eucharist, faith is required, as [Pope] Paul VI stressed: "In the first place we want to remind you that the Eucharist is a very great mystery; strictly speaking, to use the words of sacred liturgy, it is `the mystery of faith'. This is something well known to you but it is essential to the purpose of rejecting any poisonous rationalism. Many martyrs have witnessed to it with their blood. Distinguished Fathers and Doctors of the Church in unbroken succession have taught and professed it. [...] We must, therefore, approach this mystery, above all, with humble reverence, not following human arguments, which ought to be hushed, but in steadfast adherence to divine revelation" ("Mysterium Fidei").
35. Going to Jesus means believing in Him, for it is through faith that we approach our Lord. Jesus uses the metaphor of food and drink to show that He is the one who really meets all man's noblest aspirations: "How beautiful is our Catholic faith! It provides a solution for all our anxieties, calms our minds and fills our hearts with hope" ([St] J. Escriva, "The Way", 582).
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.