Nicodemus is only mentioned in the gospel account written by John. He was "a ruler of the Jews"--likely a member of the Sanhedrin in addition to being a Pharisee. His character is thought by some to be rather cowardly, and there is evidence to suggest this. In John 7:50-52, Nicodemus defends Jesus but does not commit himself as one interested in the Christ. Nicodemus is also mentioned in 19:38,39; he brought spices for the body of Jesus, but only after Joseph of Arimathea had secured the body.
Here in John 3, Nicodemus came to Jesus at night. This was likely done in order to avoid the hostility of his colleagues. Of course, to be fair, it should also be mentioned that perhaps he visited Jesus at night in order to have a more personal and uninterrupted dialogue with Him. Ultimately, the text doesn't reveal why he came to Jesus at night, and thus, any conclusion drawn is only speculation.
The miracles that Jesus had performed in Jerusalem at the Passover (cf. 2:23) convinced Nicodemus (and others) of the truth that Jesus was a teacher from God (cf. 7:16). However, this is a rather insignificant statement in view of who Jesus really was (i.e., the Word of God; 1:1,14). After all, John the baptizer, a mere man, had also been sent from God (cf. 1:6).
In John 3:3, Jesus didn't answer the words of Nicodemus but his thoughts (cf. 2:25). Evidently, Nicodemus had come to Jesus in order to learn about the approaching kingdom of God (cf. Matt.3:2). Jesus brought up the subject by declaring that no one can see the kingdom of God unless they are born again. Those not born again will not "see" the kingdom of God because they are looking for a physical kingdom, when in fact, God's kingdom is spiritual (cf. Luke 17:20,21). Thus, those who have not been reborn spiritually will never recognize the kingdom of God.
Nicodemus knew of but one birth, the fleshly one, and he failed to understand that Jesus was talking about spiritual rebirth. Therefore, he asked Jesus how an old man could be born again.
In verse 5, Jesus added the thought that one cannot "enter" the kingdom without the new birth; that is, being "born of water and the Spirit." It is clear in the Greek, since only one preposition is used, that only one act is being considered here. Jesus is not discussing two separate actions, but rather one action composed of two elements. Being born of water is a reference to baptism (cf. 3:22) and being born of the Spirit refers to a change in one's thinking caused by the Spirit's instrument, the word of God (Eph. 6:17). One is "born of water and the Spirit" when he believes the word which the Spirit gave and is immersed in water for the forgiveness of his sins. Please study the following passages on this point: Acts 2:38, I Peter 1:22,23, Titus 3:5, Ephesians 5:26, and I Corinthians 4:15. One becomes a "new creation" or is "born again" once he is "in Christ" (II Cor. 5:17), and one must be baptized to get "into Christ" (Gal. 3:27).
Dear listeners, what Jesus declared to Nicodemus is still true today: No one can enter into God's spiritual kingdom without being born again. Do you believe the word of God, and have you been washed from your sins through baptism?