John 3:13

καὶ οὐδεὶς ἀναβέβηκεν … καταβάς. The connection is: You have not believed earthly things, much less will you believe those which are heavenly; for not only are they in their own nature more difficult to understand, but there is none to testify of them save only that One who came down out of heaven. The sentence may be paraphrased thus: No one has gone up to heaven and by dwelling there gained a knowledge of the heavenly things: One only has dwelt there and is able to communicate that knowledge He, viz., who has come down from heaven. “Presence in heaven” is considered to be the ground and qualification for communicating trustworthy information regarding “heavenly things”. Direct knowledge and personal experience of heavenly things alone justify authoritative declarations about them; as in earthly things one may expect to be believed if he can say, “we speak that we do know and testify that we have seen”. But this “presence in heaven” Jesus declares to be the qualification exclusively of one person. This person He describes as “He that came down out of heaven,” adding as a further description “the Son of Man” [who is in heaven]. This description identifies this person as Jesus Himself. He claims therefore to have a unique qualification for the declaration of truth about heavenly things, and this qualification consists in this, that He and He alone has had direct perception of heavenly things. He has been in heaven. By “heaven” it is not a locality that is indicated, but that condition which is described in the prologue as πρὸς τὸν θεόν. And when He speaks of coming down out of heaven He can only mean manifesting Himself to those who are on that lower level from which they had not been able to ascend to the knowledge of heavenly things. In short, we have here the basis in Christ's own words of the statement in the prologue that the Word was in the beginning with God, and became flesh to be a light to men. Why is ὁ υἱὸς τοῦ ἀνθρώπου introduced? It identifies the person spoken of, and it suggests that He who alone had the knowledge of heavenly things now wore human nature, was accessible, and was there for the purpose of communicating this knowledge. The words added in the T.R., ὁ ὢν ἐν τῷ οὐρανῷ, affirm that although He had come out of heaven He was still in it, and they show that a condition of being, not a locality, was meant by “heaven”.