From: John 3:1-8
The Visit of Nicodemus
 Now there was a man of the Pharisees, named Nicodemus, a ruler of the Jews.  This man came to Jesus by night and said to Him, "Rabbi, we know that You are a teacher come from God; for no one can do these things that You do, unless God is with Him."  Jesus answered him, "Truly, truly, I say to you, unless one is born anew, he cannot see the Kingdom of God."  Nicodemus said to Him, "How can a man be born when he is old? Can he enter a second time into his mother's womb and be born?"  Jesus answered, "Truly, truly, I say to you, unless one is born of water and the Spirit, he cannot enter the Kingdom of God.  That which is born of the flesh is flesh, and that which is born of the Spirit is spirit.  Do not marvel that I said to you, `You must be born anew.'  The wind blows where it wills, and you hear the sound of it, but you do not know whence it comes and whether it goes; so it is with every one who is born of the Spirit."
1-21. Nicodemus was a member of the Sanhedrin of Jerusalem (cf. John 7:50). He must also have been an educated man, probably a scribe or teacher of the Law: Jesus addresses him as a "teacher of Israel". He would have been what is called an intellectual--a person who reasons things out, for whom the search for truth is a basic part of life. He was, naturally, much influenced by the Jewish intellectual climate of his time. However, if divine things are to be understood, reason is not enough: a person must be humble. The first thing Christ is going to do in His conversation with Nicodemus is to highlight the need for this virtue; that is why He does not immediately answer his questions: instead, He shows him how far he is from true wisdom: "Are you a teacher of Israel, and yet you do not understand this?" Nicodemus needs to recognize that, despite all his studies, he is still ignorant of the things of God. As St. Thomas Aquinas comments: "The Lord does not reprove him to offend him but rather because Nicodemus still relies on his own learning; therefore He desired, by having him experience this humiliation, to make him a fit dwelling-place for the Holy Spirit" ("Commentary on St. John, in loc."). From the way the conversation develops Nicodemus obviously takes this step of humility and sits before Jesus as disciple before master. Then our Lord reveals to him the mysteries of faith. From this moment onwards Nicodemus will be much wiser than all those colleagues of his who have not taken this step.
Human knowledge, on whatever scale, is something minute compared with the truths--simple to state but extremely profound--of the articles of faith (cf. Ephesians 3:15-19; 1 Corinthians 2:9). Divine truths need to be received with the simplicity of a child (without which we cannot enter the Kingdom of Heaven); then, they can be meditated on right through one's life and studied with a sense of awe, aware that divine things are always far above our heads.
1-2. Throughout this intimate dialogue, Nicodemus behaves with great refinement: he addresses Jesus with respect and calls Him Rabbi, Master. He had probably been impressed by Christ's miracles and preaching and wanted to know more. The way he reacts to our Lord's teaching is not yet very supernatural, but he is noble and upright. His visiting Jesus by night, for fear of the Jews (cf. John 19:39) is very understandable, given his position as a member of the Sanhedrin: but he takes the risk and goes to see Jesus.
When the Pharisees tried to arrest Jesus (John 7:32), failing to do so because he had such support among the people, Nicodemus energetically opposed the injustice of condemning a man without giving him a hearing; he also showed no fear, at the most difficult time of all, by honoring the dead body of the Lord (John 19:39).
3-8. Nicodemus' first question shows that he still has doubts about Jesus (is He a prophet, is He the Messiah?); and our Lord replies to him in a completely unexpected way: Nicodemus presumed He would say something about His mission and, instead, He reveals to him an astonishing truth: one must be born again, in a spiritual birth, by water and the Spirit; a whole new world opens up before Nicodemus.
Our Lord's words also paint a limitless horizon for the spiritual advancement of any Christian who willingly lets himself or herself be led by divine grace and the gifts of the Holy Spirit, which are infused at Baptism and enhanced by the Sacraments. As well as opening his soul to God, the Christian also needs to keep at bay his selfish appetites and the inclinations of pride, if he is to understand what God is teaching him in his soul: "therefore must the soul be stripped of all things created, and of its own actions and abilities--namely, of its understanding, perception and feelings--so that, when all that is unlike God and unconformed to Him is cast out, the soul may receive the likeness of God; and nothing will then remain in it that is not the will of God and it will thus be transformed in God. Wherefore, although it is true that, as we have said, God is ever in the soul, giving it, and through His presence conserving within it, its natural being, yet He does not always communicate supernatural being to it. For this is communicated only by love and grace, which not all souls possess; and all those that posses it have it not in the same degree; for some have attained more degrees of love and others fewer. Wherefore God communicates Himself most to that soul that has progressed farthest in love; namely, that has its will in closest conformity with the will of God. And the soul that has attained complete conformity and likeness of will is totally united and transformed in God supernaturally" (St. John of the Cross, "Ascent of Mount Carmel", Book II, Chapter 5).
Jesus speaks very forcefully about man's new condition: it is no longer a question of being born of the flesh, of the line of Abraham (cf. John 1:13), but of being reborn through the action of the Holy Spirit, by means of water. This is our Lord's first reference to Christian Baptism, confirming John the Baptist's prophecy (cf. Matthew 3:11; John 1:33) that He had come to institute a baptism with the Holy Spirit.
"Nicodemus had not yet savored this Spirit and this life. [...] He knew but one birth, which is from Adam and Eve; that which is from God and the Church, he did not know; he knew only the paternity which engenders to death; he did not yet know the paternity which engenders to life. [...] Whereas there are two births, he knew only of one. One is of earth, the other is of Heaven; one is of the flesh, the other of the Spirit; one of mortality, the other of eternity; one of male and female, the other of God and the Church. But the two are each unique; neither one nor the other can be repeated" (St. Augustine, "In Ioann. Evang"., 11, 6).
Our Lord speaks of the wonderful effects the Holy Spirit produces in the soul of the baptized. Just as with the wind--when it blows we realize its presence, we hear it whistling, but we do not know where it came from, or where it will end up--so with the Holy Spirit, the Divine "Breath" ("pneuma") given us in Baptism: we do not know how He comes to penetrate our heart but He makes His presence felt by the change in the conduct of whoever receives Him.
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.