John 3:1 (ESV) Now there was a Pharisee, a man named Nicodemus who was a member of the Jewish ruling council. 2 He came to Jesus at night and said, “Rabbi, we know that you are a teacher who has come from God. For no one could perform the signs you are doing if God were not with him.”
3 Jesus replied, “Very truly I tell you, no one can see the kingdom of God unless they are born again.”
4 “How can someone be born when they are old?” Nicodemus asked. “Surely they cannot enter a second time into their mother’s womb to be born!”
5 Jesus answered, “Very truly I tell you, no one can enter the kingdom of God unless they are born of water and the Spirit. 6 Flesh gives birth to flesh, but the Spirit gives birth to spirit. 7 You should not be surprised at my saying, ‘You must be born again.’ 8 The wind blows wherever it pleases. You hear its sound, but you cannot tell where it comes from or where it is going. So it is with everyone born of the Spirit.”

“How can this be?” Nicodemus asked.
12 "I have spoken to you of earthly things and you do not believe; how then will you believe if I speak of heavenly things? 13 No one has ever gone into heaven except the one who came from heaven—the Son of Man. 14 Just as Moses lifted up the snake in the wilderness, so the Son of Man must be lifted up,   15 that everyone who believes may have eternal life in him.
16 For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life. 17 For God did not send his Son into the world to condemn the world, but to save the world through him. 19 This is the verdict: Light has come into the world, but people loved darkness instead of light because their deeds were evil. 21 But whoever lives by the truth comes into the light, so that it may be seen plainly that what they have done has been done in the sight of God. 

Nicodemus would not have wanted any of his colleagues on the Jewish ruling council to know that he had been consulting with Jesus, so he came "at night". He had investigated the reports of healings (of a blind man , a sick woman , a paralytic , as examples) and concluded that he was dealing with no ordinary man.

But Jesus' opening words throw him: what's this about being "born again"? Then Jesus challenges Nicodemus' thinking  with the introduction of a spiritual dimension. As human beings, we've all be born "of water". But Jesus challenges us to also be born "of spirit". Our entrance into the kingdom of God depends on it.

And the promise of eternal life resonates in this conversation. It is a gift offered to the world due to God's great love for us. He wants for us not to perish; he wants to save us; he wants us to live by the truth and come into the light .

But if a "teacher of Israel" had trouble with this teaching, what hope do we have? Elsewhere, Jesus answers this important question: "ask, and it will be given to you; seek, and you will find; knock, and it will be opened to you. For everyone who asks receives, and the one who seeks finds, and to the one who knocks it will be opened. (Luke 11:9,10) & here "