Verse 14 (crucifixion) And as Moses lifted up the serpent in the wilderness, even so the Son of man be lifted up:
Nicodemus personal name meaning "innocent of blood." John identifies Nicodemus as a Pharisee, "a ruler of the Jews," that is, a member of the Sanhedrin, the Jewish ruling council, & as a "teacher of Israel," that is, an authority on the interpretation of the Hebrew scripture. Nicodemus coming at night suggests his timid & his trek from the darkness of his own sin & ignorance to the light of Jesus. Nicodemus greeted Jesus with a title of respect, "Rabbi" (teacher), recognizing Him as a God sent teacher whose signs bore witness to the presence of God. Jesus replied that Nicodemus could never see the Kingdom of God without being "born again" or "born of water & Spirit." Nicodemus could only marvel at the impossibility of such a thing.
True to his name, Nicodemus defended Christ before his peers who were unaware that one of their number might have believed in him.
The reference to Nicodemus' initial coming at night highlights his later public participation in Jesus' burial. Nicodemus contribution was enough aloes & spices to prepare a King for burial, & so he did. On a deeper level it recognized that in His role as King of the Jews.
The Pharisees were the most devout of Jews. The Kingdom of God is entered, not by moral achievement, but by a transformation wrought by God. Birth into the new order is through water (referring to baptism; & the Spirit). Jesus descended from heaven to bring eternal life, (participation in God's life), through being lifted up on the cross.
Reference summary used from: An Ecumenial Study Bible, The New Oxford Annotated Bible with the Apocrypha RSV & Holman Illustrated Bible Dictionary