Now this I affirm and testify in the Lord, that you must no longer live as the Gentiles do, in the futility of their minds.  You did not so learn Christ!-- assuming that you have heard about him and were taught in him, as the truth is in Jesus.  Put off your old nature which belongs to your former manner of life and is corrupt through deceitful lusts,  and be renewed in the spirit of your minds,  and put on the new nature, created after the likeness of God in true righteousness and holiness.
17-19. The Christian, who has been configured to Christ by Baptism, is called to holiness and therefore should not lead a dissolute life alienated from God, as the Gentiles do. The "futility of their minds" has led them away from God, the source of all truth (cf. Rom 1:18-32). Hence it is that when man is put in the place of God the mind operates in a vacuum and the resulting knowledge produces nothing but mere illusion and total deceit.
As St Paul tells the Romans, people who act in that way are those "who by their wickedness suppress the truth" (Rom 1:18). It is true that the human mind is capable of recognizing God as the creator of all things; but when people give their passions full rein, their will becomes weakened; they thus suppress the truth and their minds easily tend to adopt wrong ideas. All this is a result of arrogance and pride which makes man unwilling to accept God and acknowledge his own limitations as a creature: this eventually leads to the "ignorance that is in them, due to their hardness of heart" (v. 18).
Impurity opens the way to a whole series of vices and disorders connected with greed (cf. notes on Rom 1:29-31 and Rom 1:32). The term "callous" is used to indicate that these people lost their desire to try to lead a good life and even lose their very sense of morality.
22-24. The sacred text emphasizes two basic points--one's duty to put off one's "old nature" (the "old man") and, in parallel with that, the urgent need to put on the "new nature" (the "new man"). These two expressions refer directly to the symbolism of Christian Baptism, which effects the transition from the life of sin to the life of grace, thanks to the merits of Christ (cf. Rom 6:3-11).
In Baptism we have "put on Christ" (Gal 3:27) and become "partakers of the divine nature" (2 Pet 1:4). From that moment onwards a Christian's life is so radically different that to revert to one's previous--pagan--conduct is the greatest insult we could offer the body of Christ. St Paul, therefore, is not just exhorting people to root out this or that defect, but to strip themselves of the "old nature” entirely.
The "old nature" is the carnal man, vitiated from conception by original sin and become the slave of his own passions; whereas, the "new man" has been born again through the Holy Spirit at Baptism: he is no longer ruled by sin, although he is still subject to passions which have been made unruly by sin. That is why the Apostle urges us to put off the "old nature" by fighting against disordered desires and their evil effects (cf. Rom 6:12-14; 8:5-8) and by being conscious that the renewal brought about by the Holy Spirit helps the baptized person to see each and every event in his life from a new, supernatural perspective, as befits the "new man".
The change from the old to the new nature St Paul describes in terms of creation (v. 24). It does not involve any external change, as when someone changes his clothes, but rather an inner renewal, whereby the Christian, by becoming a new creature in Jesus Christ, is enabled to practise righteousness and holiness in a manner that exceeds his natural human capacity. It is not enough, then, for one to have simply a veneer of piety. "Entering the church and venerating sacred images and crosses is not sufficient for pleasing God, just as washing one's hands does not make one clean all over. What truly pleases God is that a person flees from sin and gets rid of his stains by means of confession and penance. Let him break the chains of his faults by being humble of heart" (St Anastasius of Sinai, Sermon on the Holy Synaxis).
This inner renewal of the person is something which takes a lifetime. "The power of God is made manifest in our weakness and it spurs us on to fight, to battle against our defects, although we know that we will never achieve total victory during our pilgrimage on earth. The Christian life is a continuous beginning again each day. It renews itself over and over" (St. J. Escriva, Christ Is Passing By, 114).
The People Look for Jesus
 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 [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).