13 No one has ascended to heaven but He who came down from heaven, that is, the Son of Man who is in heaven.
Verses 12 and 13 are closely connected. Jesus first descended from His preexistent state to a new state in His incarnation. In verse 13 Jesus spoke of His descent from heaven.
that is, the Son of Man
The term “Son of Man” is a messianic title that Jesus often used of Himself. He was a person who preexisted in eternity and came to earth. Jesus, and Jesus alone, has the right to speak from the viewpoint of eternal preexistence.
who is in heaven.
There is a manuscript issue with the phrase “who is in heaven.” The older manuscripts exclude it, while the majority include it. If it is included, then it points to the eternal existence of the Son of Man. The incarnation is an addition to that state.
How can the Son of Man be “in” heaven while still living on earth? His divine nature as God was still eternally existent in heaven while He lived on earth. There is unity of both deity and humanity in the person of Christ.
Verse 13 says nothing about Jesus in His humanity ascending into heaven. His humanity would ascend after His resurrection, but the point of this verse is that Christ was in heaven while He spoke. He had not left heaven in terms of His deity even though he was in His humanity. He was both on earth and in heaven at the same time. This was an unabashed claim of deity to Nicodemus and his ilk.
When Jehovah revealed Himself in the burning bush, He did not remove Himself from heaven. Every time there is a theophany does not mean that God ceases to exist everywhere.
Jesus was in heaven and on earth at the same time.
Jesus did not come down to earth in His full deity. He voluntarily set aside the use of His incommunicable attributes, such as being everywhere present, all knowing and all powerful. He came with His communicable attributes, such as love, justice, and truth. Christ could not lay aside His deity, but He could limit the use of His incommunicable attributes so that he functioned in true humanity (Php 2:5-8).
Since Jesus has resided in heaven for eternity, only He can truly communicate what it is about. It is humanly impossible to explain heaven from the human, finite viewpoint.