From: John 3:13-17
The Visit of Nicodemus (Continuation)
(Jesus said to Nicodemus,)  "No one has ascended into Heaven but He who descended from Heaven, the Son of Man.  And as Moses lifted up the serpent in the wilderness, so must the Son of Man be lifted up,  that whoever believes in Him may have eternal life."  For God so loved the world that He gave His only Son, that whoever believes in Him should not perish but have eternal life.  For God sent the Son into world, not to condemn the world, but that the world might be saved through Him.
13. This is a formal declaration of the divinity of Jesus. No one has gone up into Heaven and, therefore, no one can have perfect knowledge of God's secrets, except God Himself who became man and came down from Heaven--Jesus, the second Person of the Blessed Trinity, the Son of Man foretold in the Old Testament (cf. Daniel 7:13), to whom has been given eternal lordship over all peoples.
The Word does not stop being God on becoming man: even when He is on earth as man, He is in Heaven as God. It is only after the Resurrection and the Ascension that Christ is in Heaven as man also.
14-15. The bronze serpent which Moses set up on a pole was established by God to cure those who had been bitten by the poisonous serpents in the desert (cf. Numbers 21:8-9). Jesus compares this with His crucifixion, to show the value of His being raised up on the cross: those who look on Him with faith can obtain salvation. We could say that the good thief was the first to experience the saving power of Christ on the cross: he saw the crucified Jesus, the King of Israel, the Messiah, and was immediately promised that he would be in Paradise that very day (cf. Luke 23:39-43).
The Son of God took on our human nature to make known the hidden mystery of God's own life (cf. Mark 4:11; John 1:18; 3:1-13; Ephesians 3:9) and to free from sin and death those who look at Him with faith and love and who accept the cross of every day.
The faith of which our Lord speaks is not just intellectual acceptance of the truths He has taught: it involves recognizing Him as Son of God (cf. 1 John 5:1), sharing His very life (cf. John 1:12) and surrendering ourselves out of love and therefore becoming like Him (cf. John 10:27; 1 John 3:2). But this faith is a gift of God (cf. John 3:3, 5-8), and we should ask Him to strengthen it and increase it as the Apostles did: Lord "increase our faith!" (Luke 17:5). While faith is a supernatural, free gift, it is also a virtue, a good habit, which a person can practise and thereby develop: so the Christian, who already has the divine gift of faith, needs with the help of grace to make explicit acts of faith in order to make this virtue grow.
16-21. These words, so charged with meaning, summarize how Christ's death is the supreme sign of God's love for men (cf. the section on charity in the "Introduction to the Gospel according to St. John": pp. 31ff above). "For God so loved the world that He gave His only Son for its salvation. All our religion is a revelation of God's kindness, mercy and love for us. `God is love' (1 John 4:16), that is, love poured forth unsparingly. All is summed up in this supreme truth, which explains and illuminates everything. The story of Jesus must be seen in this light. `(He) loved me, St. Paul writes. Each of us can and must repeat it for himself--`He loved me, and gave Himself for me'(Galatians 2:20)" (Paul VI, "Homily on Corpus Christi", 13 June 1976).
Christ's self-surrender is a pressing call to respond to His great love for us: "If it is true that God has created us, that He has redeemed us, that He loves us so much that He has given up His only-begotten Son for us (John 3:16), that He waits for us--every day!--as eagerly as the father of the prodigal son did (cf. Luke 15:11-32), how can we doubt that He wants us to respond to Him with all love? The strange thing would be not to talk to God, to draw away and forget Him, and busy ourselves in activities which are closed to the constant promptings of His grace" ([St] J. Escriva, "Friends of God", 251).
"Man cannot live without love. He remains a being that is incomprehensible for himself, his life is senseless, if love is not revealed to him, if he does not encounter love, if he does not experience it and make it his own, if he does not participate intimately in it. This [...] is why Christ the Redeemer `fully reveals man to himself'. If we may use the __expression, this is the human dimension of the mystery of the Redemption. In this dimension man finds again the greatness, dignity and value that belong to his humanity.[...] The one who wishes to understand himself thoroughly [...] must, with his unrest and uncertainty and even his weakness and sinfulness, with his life and death, draw near to Christ. He must, so to speak, enter into Him with all his own self, he must `appropriate' and assimilate the whole of the reality of the Incarnation and Redemption in order to find himself. If this profound process takes place within him, he then bears fruit not only of adoration of God but also of deep wonder at himself.
How precious must man be in the eyes of the Creator, if he `gained so great a Redeemer', ("Roman Missal, Exultet" at Easter Vigil), and if God `gave His only Son' in order that man `should not perish but have eternal life'. [...]
`Increasingly contemplating the whole of Christ's mystery, the Church knows with all the certainty of faith that the Redemption that took place through the Cross has definitively restored his dignity to man and given back meaning to his life in the world, a meaning that was lost to a considerable extent because of sin. And for that reason, the Redemption was accomplished in the paschal mystery, leading through the Cross and death to Resurrection" (John Paul II, "Redemptor Hominis", 10).
Jesus demands that we have faith in Him as a first prerequisite to sharing in His love. Faith brings us out of darkness into the light, and sets us on the road to salvation. "He who does not believe is condemned already" (verse 18).
"The words of Christ are at once words of judgment and grace, of life and death. For it is only by putting to death that which is old that we can come to newness of life. Now, although this refers primarily to people, it is also true of various worldly goods which bear the mark both of man's sin and the blessing of God.[...] No one is freed from sin by himself or by his own efforts, no one is raised above himself or completely delivered from his own weakness, solitude or slavery; all have need of Christ, who is the model, master, liberator, savior, and giver of life. Even in the secular history of mankind the Gospel has acted as a leaven in the interests of liberty and progress, and it always offers itself as a leaven with regard to brotherhood, unity and peace" (Vatican II, "Ad Gentes", 8).
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.