Who hath gone up into heaven, and taken her, and brought her down from the clouds? (Baruch 3:29)
And no man hath ascended up to heaven, but he that came down from heaven, even the Son of man which is in heaven. (Joh 3:13)
Baruch is speaking here of Wisdom, which dwells in the heavens, and which is therefore unobtainable to humanity (in an ultimate sense, of course.) Wisdom is often personified in the Old Testament, and Christians understand Wisdom to be an adumbration of Christ — that is to say, Wisdom is an allegory of Christ. John is drawing our attention to the connection between the passage in Baruch, which then makes the allegorical connection between Wisdom and Christ plain. Thus while no one could ascend into heaven and bring Wisdom down to earth, the Son of God could come to earth, become one with us, and then ascend into heaven, thereby opening the pathway for us to attain Wisdom, which is Christ Himself.
 Scholars disagree as to whether Jesus answer to Nicodemus, which begins at verse ten, continues through to verse 21. Some hold that it does, while others believe that the majority of this passage is John’s commentary on Jesus’ words. The use of the conjunction “and” to begin sentences is consistent with the way Hebrew uses “and” to connect clauses, suggesting verse 10-21 may well be a single unbroken speech.