From: John 6:60-69
The Disciples' Reaction
 Many of His (Jesus') disciples, when they heard of it, said, "This is a hard saying; who can listen to it?"  But Jesus, knowing in Himself that His disciples murmured at it, said to them, "Do you take offense at this?  Then what if you were to see the Son of Man ascending where He was before?  It is the Spirit that gives life, the flesh is of no avail; the words that I have spoken to you are spirit and life.  But there are some of you that do not believe." For Jesus knew from the first who those were that did not believe, and who it was that should betray Him.  And He said, "This is why I told you that no one can come to Me unless it is granted him by the Father."
 After this many of the disciples drew back and no longer went with Him.  Jesus said to the Twelve, "Will you also go away?"  Simon Peter answered Him, "Lord, to whom shall we go? You have the words of eternal life;  and we have believed, and have come to know, that You are the Holy One of God."
60-62. Many of His listeners find the Eucharistic mystery completely incomprehensible. Jesus Christ requires His disciples to accept His words because it is He who has spoken them. That is what the supernatural act of faith involves--that act "whereby, inspired and assisted by the grace of God, we believe that the things which He has revealed are true; not because of the intrinsic truth of the things, viewed by the natural light of reason, but because of the authority of God Himself who reveals them, and who can neither be deceived nor deceive" (Vatican I, "Dei Filius", Chapter 3).
As on other occasions, Jesus speaks about future events to help His disciples believe: "I have told you before it takes place, so that when it does take place, you may believe" (John 14:29).
63. Jesus says that we cannot accept this mystery if we think of it in too human a way, in other words, by just seeking to indulge our senses or having too earthbound a view of things. Only someone who listens to His words and receives them as God's revelation, which is "spirit and life", is in a position to accept them.
66. The promise of the Eucharist, which caused arguments (verse 52) among Christ's hearers at Capernaum and scandalized some of them (verse 61), led many people to give up following Him. Jesus had outlined a wonderful and salvific truth, but those disciples closed themselves to divine grace; they were not ready to accept anything which went beyond their very limited horizons. The mystery of the Eucharist does call for a special act of faith. St. John Chrysostom therefore advised Christians: "Let us in everything believe God, and gainsay Him in nothing, though what it said be contrary to our thoughts and senses. [...] Let us act likewise in respect to the [Eucharistic] mysteries, not looking at the things set before us, but keeping in mind His words. For His words cannot deceive" (St. John Chrysostom, "Hom. on St. Matthew", 82).
67-71. This passage is similar to that at Capernaum where Peter again, in the name of the Twelve, takes the initiative in expressing his faith in Jesus as Messiah (cf. Matthew 16:13-20; Mark 8:27-30). Other people present may have been unbelieving, but the Apostles are not scandalized by our Lord's words: they say that they have already a deep-rooted confidence in Him; they do not want to leave Him. What St. Peter says (verse 68) is not just a statement of human solidarity but an _expression of genuine supernatural faith--as yet imperfect--which is the result of the influence of divine grace on his soul (cf. Matthew 16:17).
Although the Twelve stay with Him at this point, Judas will later betray the Master. Jesus' foreknowledge of this future infidelity throws a shadow over His joy at the loyalty of the Twelve. We Christians should be humble enough to realize that we are capable of betraying our Lord if we give up using the means He has left us to cleave to Him. St. Peter's words (verse 68) are a beautiful aspiration we can use whenever we feel tempted.
68. Simon Peter expresses the feelings of the Apostles who, through staying loyal to Jesus, are getting to know Him much better and becoming more closely involved with Him: "Seek Jesus; endeavoring to acquire a deep personal faith that will inform and direct your whole life. But, above all, let it be your commitment and your program to love Jesus, with a sincere, authentic and personal love. He must be your friend and your support along the path of life. He alone has words of eternal life" ([Pope] John Paul II, "Address to Students in Guadalajara", 30 January 1979).
69. "The Holy One of God": this is what the original text must have said, according to most of the Greek codexes and the most important early translations. "The Holy One" is one of the expressions which designate the Messiah (cf. Mark 1:24; Luke 1:35; 4:34; Acts 2:27; Psalm 16:10), or God Himself (cf. Isaiah 6:3; 43:15; 1 Peter 1:15; 1 John 2:20; etc.). The rendering "the Christ, the Son of God" found in some translations, including the Vulgate, is supported by less important Greek manuscripts, and would seem to be an explanation of the messianic significance of the original phrase.
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.