Jesus asks this to show Nicodemus where unbelief leads. The new birth was the simpler lesson, and Jesus had explained it with pictures from everyday life: birth, water, and wind (John 3:3-8). If that was not believed, deeper truths would not be believed either. Barnes says the "earthly things" were not worldly things but works of the Spirit that happen on earth, "the plainest and most obvious of the doctrines of religion." The "heavenly things" are, in Barnes' words, "the more profound and inscrutable things pertaining to the redemption of men."

Nicodemus does not reply. Jesus goes on to tell him the heavenly things. No man has ascended up to heaven "but he that came down from heaven, even the Son of man" (John 3:13). He must "be lifted up" as Moses lifted up the serpent (John 3:14). And "God so loved the world, that he gave his only begotten Son" (John 3:16). The question opens the door to the most famous words in John's Gospel.

Gill says the earthly things were either the easier doctrines of the Gospel or truths delivered "in a plain and easy style, and illustrated by similes taken from earthly things, as from human birth, from the water, and from the wind." By heavenly things he understands deeper truths, including "the descent of the Messiah from heaven" and His death, "signified by the lifting up of the serpent on a pole in the wilderness."

That picture comes from Numbers 21:9. When the Israelites were bitten by serpents in the wilderness, Moses made a serpent of brass and put it on a pole, and whoever looked at it lived. Jesus uses this event in John 3:14 to describe His own lifting up.

Jesus is the only one who can speak of heaven from firsthand knowledge. He says, "no man hath ascended up to heaven, but he that came down from heaven, even the Son of man" (John 3:13). Later in the same chapter the point is made again: "he that cometh from heaven is above all" (John 3:31). The question shows that faith is not a reward for figuring everything out. It is trusting the One who has seen what we have not.

The heavenly things Jesus then reveals are full of grace. The Son is lifted up "that whosoever believeth in him should not perish" (John 3:15). God "sent not his Son into the world to condemn the world" (John 3:17). The warning in the question leads straight into an invitation.