From: John 6:44-51
The Discourse on the Bread of Life (Continuation)
(Jesus said to the Jews,)  "No one can come to Me unless the Father who sent Me draws him; and I will raise him up on the last day.  It is written in the prophets, `And they shall all be taught by God.' Every one who has heard and learned from the Father comes to Me.  Not that any one has seen the Father except Him who is from God; He has seen the Father.  Truly, truly, I say to you, he who believes has eternal life.  I am the bread of life.  Your fathers ate the manna in the wilderness, and they died.  This is the bread which comes down from Heaven, that a man may eat of it and not die.  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."
44-45. Seeking Jesus until one finds Him is a free gift which no one can obtain through his own efforts, although everyone should try to be well disposed to receiving it. The Magisterium of the Church has recalled this teaching in Vatican II: "Before this faith can be exercised, man must have the grace of God to move and assist him; he must have the interior help of the Holy Spirit, who moves the heart and converts it to God, who opens the eyes of the mind and makes it easy for all to accept and believe the truth" ("Dei Verbum", 5).
When Jesus says, "They shall all be taught by God", He is invoking Isaiah 54:13 and Jeremiah 31:33ff, where the prophets refer to the future Covenant which God will establish with His people when the Messiah comes, the Covenant which will be sealed forever with the blood of the Messiah and which God will write on their hearts (cf. Isaiah 53:10-12; Jeremiah 31:31-34).
The last sentence of verse 45 refers to God's Revelation through the prophets and especially through Jesus Christ.
46. Men can know God the Father only through Jesus Christ, because only He has seen the Father, whom He has come to reveal to us. In his prologue St. John already said: "No one has ever seen God; the only Son, who is in the bosom of the Father, He has made Him known" (John 1:18). Later on Jesus will say to Philip at the Last Supper: "He who has seen Me has seen the Father" (John 14:9), for Christ is the Way, the Truth and the Life, and no one goes to the Father except through Him (cf. John 14:6).
In other words, in Christ God's revelation to men reaches its climax: "For He sent His Son, the eternal Word who enlightens all men, to dwell among men and to tell them about the inner life of God (cf. John 1:1-18). Hence, Jesus Christ, sent as `a man among men', `utters thewords of God' (John 3:34), and accomplishes the saving work which the Father gave Him to do (cf. John 5:36; 17:4). To see Jesus is to see His Father (cf. John 14:9)" (Vatican II, "Dei Verbum", 4).
48. With this solemn declaration, which He repeats because of His audience's doubts, (cf. John 6:35, 41, 48), Jesus begins the second part of His discourse, in which He explicitly reveals the great mystery of the Blessed Eucharist. Christ's words have such a tremendous realism about them that they cannot be interpreted in a figurative way: if Christ were not really present under the species of bread and wine, this discourse would make absolutely no sense. But if His real presence in the Eucharist is accepted on faith, then His meaning is quite clear and we can see how infinite and tender His love for us is.
This is so great a mystery that it has always acted as a touchstone for Christian faith: it is proclaimed as "the mystery of our faith" immediately after the Consecration of the Mass. Some of our Lord's hearers were scandalized by what He said on this occasion (cf. verses 60-66). Down through history people have tried to dilute the obvious meaning of our Lord's words. In our own day the Magisterium of the Church has explained this teaching in these words" "When Transubstantiation has taken place, there is no doubt that the appearance of the bread and the appearance of the wine take on a new expressiveness and a new purpose since they are no longer common bread and common drink, but rather the sign of something sacred and the sign of spiritual food. But they take on a new expressiveness and a new purpose for the very reason that they contain a new `reality' which we are right to call "ontological". For beneath these appearances there is no longer what was there before but something quite different [...] since on the conversion of the bread and wine's substance, or nature, into the body and blood of Christ, nothing is left of the bread and the wine but the appearances alone. Beneath these appearances Christ is present whole and entire, bodily present too, in His physical `reality', although not in the manner in which bodies are present in place.
For this reason the Fathers have had to issue frequent warnings to the faithful, when they consider this august Sacrament, not to be satisfied with the senses which announce the properties of bread and wine. They should rather assent to the words of Christ: these are of such power that they change, transform, `transelement' the bread and the wine into His body and blood. The reason for this, as the same Fathers say more than once, is that the power which performs this action is the same power of Almighty God that created the whole universe out of nothing at the beginning of time" (Paul VI, "Mysterium Fidei").
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 to the 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).
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.