Nicodemus was a Pharisee and a ruler of the people of Israel during the time of Jesus. He was a man who had many scripture passages memorized and was a part of the Sanhedrin or ruling counsel. He should have understood the concept about the new birth since it is mentioned in the Old Testament. However, Nicodemus was trusting in the fact that he had been born a Jew for entry into heaven. To this, Jesus begins his teaching to tell him he needs, as does everyone else, to be born again. This was astonishing to someone who already thought he had eternal life guaranteed. That a person must be born again is mentioned in numerous parts of the New Testament in 1 Peter 1:3; Titus 3:5, Romans 6:1-11, Galatians 6:15, James 1:18.
For Jesus, it is not simply a matter of being reformed morally to enter into heaven or rule keeping. Our good deeds are like filthy rags to God and we can never be good enough by the flesh. Later in verse 6, Jesus will say in essence "flesh is flesh" and it cannot earn entry into heaven. Nicodemus understood rule keeping and works. The Bible makes it clear that is it not something of works that we should boast Ephesians 2:9. It is a matter of regeneration that occurs by the work of the Spirit not of man.
4 Nicodemus *said to Him, “How can a man be born when he is old? He cannot enter a second time into his mother’s womb and be born, can he?” 5 Jesus answered, “Truly, truly, I say to you, unless one is born of water and the Spirit he cannot enter into the kingdom of God. 6 That which is born of the flesh is flesh, and that which is born of the Spirit is spirit. 7 Do not be amazed that I said to you, ‘You must be born [c]again.’8 The wind blows where it wishes and you hear the sound of it, but do not know where it comes from and where it is going; so is everyone who is born of the Spirit.” vs.4-8
Nicodemus responds in astonishment. He takes Jesus words literally in verse 4. To emphasize his point, the words "truly, truly" mean that Jesus is being very emphatic about what he says. Jesus says to him that you think it is enough to be born a Jew. But, you must be also born of the Spirit. You don't understand the Spirit anymore than you understand the wind vs.8, but that doesn't mean you can't experience it. The idea of being born of "water" is the natural birth that we all experience.
9 "Nicodemus said to Him, “How can these things be?” 10 Jesus answered and said to him, “Are you the teacher of Israel and do not understand these things? 11 Truly, truly, I say to you, we speak of what we know and testify of what we have seen, and you do not accept our testimony. 12 If I told you earthly things and you do not believe, how will you believe if I tell you heavenly things? 13 No one has ascended into heaven, but He who descended from heaven: the Son of Man."
Jesus is saying that as the sole person who has descended from heaven he has the authority to teach this. As one who witnessed it and knows it he can teach it. Jesus is amazed that Nicodemus doesn't immediately understand what he is saying vs.10. The fact that Nicodemus had so many scriptures memorized and appeared just in everyone else's eyes should cause us to pause and consider these words of Jesus.
