John 3: 9-15, Second Birth

Nicodemus has asked Jesus about his teachings, to which Jesus has responded, "You must be born a second time" (or "born from above".)  This teaching follows an Old Testament prophesy, recorded in Ezekiel 36: 26-27.  But all of this is foreign to Nicodemus.

John 3: 9-12, This strange rebirth
“How can this be?” Nicodemus asked.

10 “You are Israel’s teacher,” said Jesus, “and do you not understand these things?
11 Very truly I tell you, we speak of what we know, and we testify to what we have seen, but still you people do not accept our testimony. 
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? 

Nicodemus persists and Jesus challenges him, as a religious leader, to put the spiritual birth in context. If this event is late in Jesus' ministry, then most of the religious leaders have already rejected Jesus (which explains the night visit) and so have rejected the testimony of the actions and signs provided by Jesus.

I am unsure of the point of verse 12. Do the "earthly things" refer to Jesus' comments about the wind?

John 3: 13-15, The snake of Moses

 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.”

The divine Messiah ("Son of Man") comes from heaven and will return there. (The NIV footnotes say that some ancient manuscripts record, in verse 13, "the Son of Man, who is in heaven.")

In Numbers 21: 8-9, Moses is told by God to create a bronze image of a snake and place it on a pole. Then anyone who had been bitten by a poisonous snake could look up at the image and God would heal that person. Here Jesus is represented as the bronze snake and when people look to him, their "poison" is taken away.

It will not be clear when the words of Jesus end and the commentary of the gospel writer begins. The gospel writer, John, will repeatedly emphasize belief in the Messiah as the doorway to eternal life.