John 3:12,13

If I have told you earthly things As the truths which I have taught you concerning the spiritual nature of God's kingdom, and the qualifications of his subjects, may properly be termed, because they are capable of being represented to you in a familiar way, and of being illustrated by such obvious and well-known similitudes as to be rendered thereby perfectly plain and easy to be understood. Or, by earthly things, he might mean things to be experienced and enjoyed on earth, such as the new birth and the present privileges of the children of God. And ye believe not Even these; how shall ye believe if I tell you of heavenly things If I should go on to teach you other doctrines, much more mysterious and sublime, and not capable of being thus illustrated and explained? Our Lord has been thought by some to refer here to those sublime and heavenly doctrines which were afterward revealed, such as the eternity of the Son, the fulness of the Godhead dwelling in him bodily, the unity of the Father, Son, and Spirit, and those other mysteries of godliness that are above the reach of human reason, and cannot be illustrated by earthly things. It is more probable, however, as Dr. Doddridge observes, that he more immediately refers “to the doctrines which he mentions in the remaining part of his discourse to Nicodemus of his descent from heaven to instruct us in the things of God, and be united to the human nature here below, while, by his divine nature, he still continued to be present above, of the design for which he came into the world, to be lifted up upon the cross, that he might save us from our sins, of everlasting life, and happiness to be obtained by faith in his death, and of the condemnation of all those that should reject him; which may be counted as the deep things of God, which he reveals unto us by his Spirit, and which the natural man, who disregards that Spirit, receiveth not, for they are foolishness unto him, 1Co 2:10; 1 Corinthians 2:14.” And no man, &c. As if he had said, For here you must rely on my single testimony, whereas in the other points, termed earthly things, you have a cloud of witnesses. Or the connection and sense may be, Yet the truth of my doctrine concerning these heavenly things you will have no just reason to suspect, considering whence it comes, and who it is that reveals it. For no man hath ascended up to heaven To search into the secret counsels of God, and to obtain an intimate and perfect knowledge of his mind and will; but he that came down from heaven Qualified and commissioned in the most extraordinary manner to reveal them, as far as is proper, to mankind; even the Son of man which is in heaven Is present there by his divine nature, which fills both heaven and earth, even while he is here on earth as to his human nature. This is a plain instance of what is usually termed the communication of properties between Christ's divine and human nature, whereby what is proper to the divine nature is spoken concerning the human; and what is proper to the human, is spoken of the divine. “Beza, and some others, suppose that the present tense, (ων, who is,) is here put for the past, (ην, w ho was,) of which construction we have some examples, particularly John 9:25. Accordingly they translate the clause, The Son of man, who was in heaven: but the common translation may be retained, [and interpreted,] thus: Moses, your lawgiver, did not ascend into heaven; he only went up to mount Sinai, and that but for a few days, that he might receive the law from God. Whereas, the Son of man, (this was one of the Messiah's titles,) who is come down from heaven, (ο εκ του ουρανου καταβας,) who is commissioned by God in an extraordinary manner, to reveal his will to men, and in respect of whose commission, all the other messengers of God may be said to have been of the earth: (see John 3:31; Hebrews 12:25:) he hath ascended up to heaven Hath received the clearest and most extensive views of spiritual things; hath penetrated into the recesses of the divine counsels; (see Proverbs 30:3;) nay, is, at present, in heaven, is with God, is conscious of all his gracious purposes toward men, consequently must be a messenger of much higher dignity than Moses, or Elijah, or any of the prophets, for whom you entertain so great a regard.” Macknight.