From: John 6:51-58
The Discourse on the Bread of Life (Continuation)
(Jesus said to the Jews,)  "I am the living bread which came down from Heaven; if any one eats of this bread, he will live for ever; and the bread which I shall give for the life of the world is My flesh."  The Jews disputed among themselves, saying, "How can this Man give us His flesh to eat?"  So Jesus said to them, "Truly, truly, I say to you, unless you eat the flesh of the Son of Man and drink His blood, you have no life in you;  he who eats My flesh and drinks My blood has eternal life, and I will raise him up on the last day.  For My flesh is food indeed, and My blood is drink indeed.  He who eats My flesh and drinks My blood abides in Me, and I in him.  As the living Father sent Me, and I live because of the Father, so he who eats Me will live because of Me.  This is the bread which came from Heaven, not such as the fathers ate and died; he who eats this bread will live for ever."
49-51. The manna during the Exodus was a figure of this bread--Christ Himself--which nourishes Christians on their pilgrimage through this world. Communion is the wonderful banquet at which Christ gives Himself to us: "the bread which I shall give for the life of the world is My flesh". These words promise the manifestation of the Eucharist at the Last Supper: "This is My body which is for you" (1 Corinthians 11:24). The words "for the life of the world" and "for you" refer tothe redemptive value of the sacrifice of Christ on the cross. In some sacrifices of the Old Testament, which were a figure of the sacrifice of Christ, part of the animal offered up was later used for food, signifying participation in the sacred rite (cf. Exodus 11:3-4). So, by receiving Holy Communion, we are sharing in the sacrifice of Christ: which is why the Church sings in the Liturgy of the Hours on the Feast of Corpus Christi: "O sacred feast in which we partake of Christ: His sufferings are remembered, our minds are filled with His grace and we receive a pledge of the glory that is to be ours" ("Magnificat Antiphon", Evening Prayer II).
52. Christ's hearers understand perfectly well that He means exactly what He says; but they cannot believe that what He says could be true; if they had understood Him in a metaphorical, figurative or symbolic sense there would be no reason for them to be surprised and nothing to cause an argument. Later, Jesus reaffirms what He has said--confirming what they have understood Him to say (cf. verses 54-56).
53. Once again Jesus stresses very forcefully that it is necessary to receive Him in the Blessed Eucharist in order to share in divine life and develop the life of grace received in Baptism. No parent is content to bring children into the world: they have to be nourished and looked after to enable them to reach maturity. "We receive Jesus Christ in Holy Communion to nourish our souls and to give us an increase of grace and the gift of eternal life" ("St. Pius X Catechism", 289).
54. Jesus clearly states that His body and blood are a pledge of eternal life and a guarantee of the resurrection of the body. St. Thomas Aquinas gives this explanation: "The Word gives life to our souls, but the Word made flesh nourishes our bodies. In this Sacrament is contained the Word not only in His divinity but also in His humanity; therefore, it is the cause not only of the glorification of our souls but also of that of our bodies" ("Commentary on St. John, in loc.").
Our Lord uses a stronger word than just "eating" (the original verb could be translated as "chewing") which shows that Communion is a real meal. There is no room for saying that He was speaking only symbolically, which would mean that Communion was only a metaphor and not really eating and drinking the Body and Blood of Christ.
"All these invitations, promises and threats sprang from the great desire which (Jesus) had of giving us Himself in the holy Sacrament of the altar. But why should Jesus so ardently desire us to receive Him in Holy Communion? It is because love always sighs for, and tends to a union with, the object beloved. True friends wish to be united in such a manner as to become only one. The love of God for us being immense, He destined us to possess Him not only in Heaven, but also here below, by the most intimate union, under the appearance of bread in the Eucharist. It is true we do not see Him; but He beholds us, and is really present; yes, He is present in order that we may possess Him and He conceals Himself, that we may desire Him, and until we reach our true homeland Jesus Christ wishes in this way to be entirely ours, and to be perfectly united to us" (St. Alphonsus Liguori, "The Love of Our Lord Jesus Christ Reduced to Practice", Chapter 2).
55. In the same way as bodily food is necessary for life on earth, Holy Communion is necessary for maintaining the life of the soul, which is why the Church exhorts us to receive this Sacrament frequently: "Every day, as is desirable, and in the greatest possible numbers, the faithful must take an active part in the sacrifice of the Mass, avail themselves of the pure, holy refreshment of Holy Communion and make a suitable thanksgiving in return for this great gift of Christ the Lord. Here are the words they should keep in mind: `Jesus Christ and the Church desire all Christ's faithful to approach the sacred banquet every day. The basis of this desire is that they should be united to God by the sacrament and draw strength from it to restrain lust, to wash away the slight faults of daily occurrence and to take precautions against the more serious sins to which human frailty is liable' (Decree of the S.C. of the Council, 20 December 1905)" ([Pope] Paul VI, "Mysterium Fidei").
"The Savior has instituted the most august sacrament of the Eucharist, which truly contains His flesh and His blood, so that he who eats this bread may live forever; whosoever, therefore, makes use of it often with devotion so strengthens the health and the life of his soul, that it is almost impossible for him to be poisoned by any kind of evil affection. We cannot be nourished with this flesh of life, and live with the affections of death. [...]. Christians who are damned will be unable to make any reply when the just Judge shows them how much they are to blame for dying spiritually, since it was so easy for them to maintain themselves in life and in health by eating His Body which He had left them for this purpose. Unhappy souls, He will say, why did you die, seeing that you had at your command the fruit and the food of life?" (St. Francis de Sales, "Introduction to the Devout Life", II, 20, 1).
56. The most important effect of the Blessed Eucharist is intimate union with Jesus Christ. The very word "communion" suggests sharing in the life of our Lord and becoming one with Him; if our union with Jesus is promoted by all the sacraments through the grace which they give us, this happens more intensely in the Eucharist, for in it we receive not only grace but the very Author of grace: "Really sharing in the body of the Lord in the breaking of the eucharistic bread, we are taken up into communion with Him and with one another. `Because the bread is one, we, though many, are one body, all of us who partake of the one bread' (1 Corinthians 10:17)" (Vatican II, "Lumen Gentium", 7). Precisely because the Eucharist is the sacrament which best signifies and effects our union with Christ, it is there that the whole Church manifests and effects its unity: Jesus Christ "instituted in His Church the wonderful sacrament of the Eucharist, by which the unity of the Church is both signified and brought about" (Vatican II, "Unitatis Reditegratio", 2).
57. In Christ, the Incarnate Word sent to mankind, "the whole fullness of deity, dwells bodily (Colossians 2:9) through the ineffable union of His human nature and His divine nature in the Person of the Word. By receiving in this sacrament the body and blood of Christ indissolubly united to His divinity, we share in the divine life of the second Person of the Blessed Trinity. We will never be able to appreciate enough the intimacy with God Himself--Father, Son and Holy Spirit--that we are offered in the eucharistic banquet.
"We can therefore do nothing more agreeable to Jesus Christ than to go to Communion with the dispositions suitable to so great an action, since we are then united to Jesus Christ, according to the desire of this all-loving God. I have said with `suitable' and not `worthy' disposition, for who could communicate if it was necessary to be worthy of so great a Savior? No one but a God would be worthy to receive a God. But by this word suitable, or convenient, I mean such a disposition as becomes a miserable creature, who is clothed with the unhappy flesh of Adam. Ordinarily speaking, it is sufficient that we communicate in a state of grace and with an anxious desire of advancing in the love of Jesus Christ" (St. Alphonsus Liguori, "The Love of Our Lord Jesus Christ Reduced to Practice", Chapter 2).
58. For the third time (cf. 6:31-32 and 6:49) Jesus compares the true bread of life, His own body, with the manna God used to feed the Israelites every day during their forty years in the wilderness--thereby, inviting us to nourish our soul frequently with the food of His body.
"`Going to Communion every day for so many years! Anybody else would be a saint by now, you told me, and I...I'm always the same!' Son, I replied, keep up your daily Communion, and think: what would I be if I had not gone'" ([St] J. Escriva, "The Way", 534).
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.