4 And He said, “It is the Spirit that quickens; the flesh profits nothing.” Before we expound this, as the Lord grants us, that other must not be negligently passed over, where He says, “Then what if you shall see the Son of man ascending where He was before?” For Christ is the Son of man, of the Virgin Mary. Therefore Son of man He began to be here on earth, where He took flesh from the earth. For which cause it was said prophetically, “Truth is sprung from the earth.” Then what does He mean when He says, “When you shall see the Son of man ascending where He was before”?
For there had been no question if He had spoken thus: “If you shall see the Son of God ascending where He was before.” But since He said, “The Son of man ascending where He was before,” surely the Son of man was not in heaven before the time when He began to have a being on earth? Here, indeed, He said, “where He was before,” just as if He were not there at this time when He spoke these words. But in another place He says, “No man has ascended into heaven but He that came down from heaven, the Son of man who is in heaven.” He said not “was,” but, says He, “the Son of man who is in heaven.”
He was speaking on earth, and He declared Himself to be in heaven. And yet He did not speak thus: “No man has ascended into heaven but He that came down from heaven,” the Son of God, “who is in heaven.” Whither tends it, but to make us understand that which even in the former discourse I commended to your minds, my beloved, that Christ, both God and man, is one person, not two persons, lest our faith be not a trinity, but a quaternity? Christ, therefore, is one; the Word, soul and flesh, one Christ; the Son of God and Son of man, one Christ; Son of God always, Son of man in time, yet one Christ in regard to unity of person.
In heaven He was when He spoke on earth. He was Son of man in heaven in that manner in which He was Son of God on earth; Son of God on earth in the flesh which He took, Son of man in heaven in the unity of person.
Source: Tractates on the Gospel of John (New Advent)