8 And He goes on: “And no man has ascended into heaven, but He that came down from heaven, the Son of man who is in heaven.” Behold, He was here, and was also in heaven; was here in His flesh, in heaven by His divinity; yea, everywhere by His divinity. Born of a mother, not quitting the Father. Two nativities of Christ are understood: one divine, the other human: one, that by which we were to be made; the other, that by which we were to be made anew: both marvellous; that without mother, this without father.
But because He had taken a body of Adam,— for Mary was of Adam,— and was about to raise that same body again, it was an earthly thing He had said in saying, “Destroy this temple, and in three days I will raise it up.” But this was a heavenly thing, when He said, “Unless a man be born again of water and of the Spirit, he shall not see the kingdom of God.” Come then, brethren! God has willed to be the Son of man; and willed men to be sons of God. He came down for our sakes; let us ascend for His sake.
For He alone descended and ascended, He who says, “No man has ascended into heaven, but He who came down from heaven.” Are they not therefore to ascend into heaven whom He makes sons of God? Certainly they are: this is the promise to us, “They shall be equal to the angels of God.” Then how is it that no man ascends, but He that descended? Because one only descended, only one ascends. What of the rest? What are we to understand, but that they shall be His members, that one may ascend?
Therefore it follows that “no man has ascended into heaven, but He who came down from heaven, the Son of man who is in heaven.” Do you marvel that He was both here and in heaven? Such He made His disciples. Hear the Apostle Paul saying, “But our conversation is in heaven.” If the Apostle Paul, a man, walked in the flesh on earth, and yet had his conversation in heaven, was the God of heaven and earth not able to be both in heaven and on earth?
Source: Tractates on the Gospel of John (New Advent)