If I told you earthly things and ye believe not, how will ye believe if I tell you of heavenly things? Our Lord here drops the plural form of address, and returns to the singular. He is about to refer to matters in which the testimony of disciples was not available.
It has sometimes been said that the "earthly" and "heavenly" things refer to the wind parable and its interpretation. But, on the supposition that there is a parable or metaphor in John 3:8, which we have seen reason to doubt, there would be no perplexity about the reception of the earthly illustration; none could in that day have made a moment's question touching the invisibility and incomprehensibility of the motion of the wind.
The birth from water has been supposed by others to be the ( ἐπίγειον) "earthly" thing of which he had spoken, as contrasted with the heavenly thing, the birth anew from the Spirit. But this also is improbable, for of all the things of which Jesus spoke, that was the least likely to have been rejected by the Pharisaic party.
The "earthly things" are the subject matter of the discourse as a whole, in apprehending which Nicodemus manifested such obtuseness. The change, renovation of human nature, the new beginning "from the Spirit" of each human life, was indeed operated on the ground of an earthly experience, and came fairly within the compass of common appreciation.
Though produced by the Spirit, these things were enacted on earth. When Nicodemus asks the question "how?" he launches the inquiry into another region. There is wide difference between the question "what?"
and the question "how?" The one in physical science refers to the whole range of phenomena, and the answer states the facts as they present themselves to the senses; the other question inquires into what Bacon called the latens processus—into verae causae, into the movements and method of the creative hand.
So the answer to the question "what?" may be an "earthly thing," the answer to the question "how?" a "heavenly thing." If Christ answer the "how" of his listener, he raises his mind to the "heavenly" and transcendental realities which Nicodemus and we too will have to receive on an authority which entirely outsoars that of daily experience or temporal phenomena.
Truly he does proceed to do so, but the difficulty of acceptance is indefinitely augmented. The answer of Christ to the matters of personal experience, verifiable by conscience and affirmed by Scripture, was difficult to the master of Israel.
The answer of Jesus to the question "how?" may prove far more formidable. It involves the revelation of "the Son of man," and the redemption by the cross, and the ascension of the Son of man into heaven, and the love of God to the world, and the gift of eternal life to faith.