From: John 6:24-35
The People Look for Jesus
 On the next day the people who remained on the other side of the sea saw that there had been only one boat there, and that Jesus had not entered the boat with His disciples, but that His disciples had gone away alone.  However, boats from Tiberias came near the place where they ate the bread after the Lord had given thanks.  So when the people saw that Jesus was not there, nor His disciples, they themselves got into the boats and went to Capernaum, seeking Jesus.
The Discourse on the Bread of Life
 When they found Him on the other side of the sea, they said to Him, "Rabbi, when did You come here?"  Jesus answered them, "Truly, truly, I say to you, you seek Me, not because you saw signs, but because you ate your fill of the loaves.  Do not labor for the food which perishes, but the food which endures to eternal life, which the Son of Man will give to you; for on Him has God the Father set His seal."  Then they said to Him, "What must we do, to be doing the works of God?"  Jesus answered them, "This is the work of God, that you believe in Him He has sent."
 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."
26. Our Lord begins by pointing out that their attitudes are wrong: if they have the right attitude they will be able to understand His teaching in the eucharistic discourse. "You seek me", St. Augustine comments, "for the flesh, not for the spirit. How many seek Jesus for no other purpose than that He may do them good in this present life! [...] Scarcely ever is Jesus sought for Jesus' sake" ("In Ioann. Evang.", 25, 10).
This verse marks the beginning of the discourse on the bread of life which goes up to verse 59. It opens with an introduction in the form of a dialogue between Jesus and the Jews (verses 26-34), in which our Lord reveals Himself as the bringer of the messianic gifts. Then comes the first part of the discourse (verses 35-47), in which Jesus presents Himself as the Bread of Life, in the sense that faith in Him is food for eternal life. In the second part (verses 48-59) Christ reveals the mystery of the Eucharist: He is the Bread of Life who gives Himself sacramentally as genuine food.
27. Bodily food helps keep us alive in this world; spiritual food sustains and develops supernatural life, which will last forever in Heaven. This food, which only God can give us, consists mainly in the gift of faith and sanctifying grace. Through God's infinite love we are given, in the Blessed Eucharist, the very author of these gifts, Jesus Christ, as nourishment for our souls.
"On Him has God the Father set His seal": our Lord here refers to the authority by virtue of which He can give men the gifts He has referred to: for, being God and man, Jesus' human nature is the instrument by means of which the Second Person of the Blessed Trinity acts. St. Thomas Aquinas comments on this sentence as follows: "What the Son of Man will give He possesses through His superiority over all other men in His singular and outstanding fullness of grace. ... When a seal is impressed on wax, the wax receives the complete form of the seal. So it is that the Son received the entire form of the Father. This occurred in two ways; eternally (eternal generation), which is not referred to here because the seal and the sealed are different in nature from one another; what is referred to here is the other manner, that is, the mystery of the Incarnation, whereby God the Father impressed on human nature the Word, who is the reflection and the very stamp of God's nature, as Hebrews 1:3 says" ("Commentary on St. John, in loc.").
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 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.