Nicodemus, a man from the Pharisees, member of the Jewish High Council (Sanhedrin) sees Jesus as a teacher / prophet sent by God, who performs special signs.
The Pharisees ('separated ones') are known in Jesus' time for their very strict interpretation and observance of the law of Moses.
Nicodemus begins with: “We know…”
He “believes” in Jesus because of seeing the signs. John 2 verse 23. He thinks he sees the Kingdom of God. However, it is a faith without depth and durability, as the sequel shows.
Jesus' answer indicates that the true vision of the Kingdom of God is only possible if a person is 'born again'. He then explains what this new birth entails.
Jesus' words only raise questions for Nicodemus: 'How can...? This is where his ignorance shows. He could have known from the Old Testament what Jesus meant. For example, Ezekiel 36 verse 25 to 27 states:
25 “I will sprinkle clean water on you, and you will be clean. I will cleanse you from all your filthiness and from all your idols.
26 Then I will give you a new heart, and a new spirit will I put within you. I will take away the heart of stone from your body and give you a heart of flesh.
27 I will put My Spirit within you. I will cause you to walk in My statutes, and to keep My judgments, and to keep them.”
Jesus therefore asks a confrontational question: “Are you the teacher of Israel, and do you not know these things?”
Like Nicodemus, He uses the words “We know…”. He refers to His heavenly descent, and that He speaks on behalf of God the Father. Jesus is the true Teacher of Israel.
Then He speaks about the purpose of His mission: The Son of Man (compare Daniel 7 verse 13, 14) must be lifted up as Moses lifted up the bronze serpent in the wilderness. Numbers 21 verse 9. Being lifted up refers to His crucifixion.
Jesus answered Nicodemus' question: How is a person born again? as follows:
“Whoever believes in the Son of Man (Jesus) has eternal life”
16 “For God so loved the world, that He gave His only begotten Son, that whoever believes in Him should not perish, but have eternal life.”
The assignment for man is not: be born again, but: believe in God's Son, Jesus.
If someone believes, he/she may conclude that he/she has been born again -by the Holy Spirit.
“For the Son of Man came not to destroy men's souls, but to save them.” Luke 9 verse 56.
Anyone who does not believe in Jesus is already condemned for that reason. (verse 18) The reason for unbelief is that someone by nature prefers darkness to light. He does not come to the Light, to Jesus, so that his 'evil works' are not exposed.
In chapter 3 verse 36 John concludes:
“He who believes in the Son has eternal life, but whoever disobeys the Son will not see life, but the wrath of God remains on him.”
In John 7 verse 50, 51 Nicodemus objects to the condemnation of Jesus, without being interrogated first.
And after Jesus is 'lifted up' on the cross, Nicodemus comes to honor Him:
“And Nicodemus (who came to Jesus at night) also came and brought a mixture of myrrh and aloes, about a hundred pounds worth.” John 19 verse 39.