16 This deep and beautiful mystery of His assumption of manhood the Lord Himself reveals in the words, No man has ascended into heaven, but He that descended from heaven, even the Son of Man which is in heaven. 'Descended from heaven' refers to His origin from the Spirit: for though Mary contributed to His growth in the womb and birth all that is natural to her sex, His body did not owe to her its origin. The 'Son of Man' refers to the birth of the flesh conceived in the Virgin; 'Who is in heaven' implies the power of His eternal nature: an infinite nature, which could not restrict itself to the limits of the body, of which it was itself the source and base.
By the virtue of the Spirit and the power of God the Word, though He abode in the form of a servant, He was ever present as Lord of all, within and beyond the circle of heaven and earth. So He descended from heaven and is the Son of Man, yet is in heaven: for the Word made flesh did not cease to be the Word. As the Word, He is in heaven, as flesh He is the Son of Man. As Word made flesh, He is at once from heaven, and Son of Man, and in heaven, for the power of the Word, abiding eternally without body, was present still in the heaven He had left: to Him and to none other the flesh owed its origin. So the Word made flesh, though He was flesh, yet never ceased to be the Word.
Source: On the Trinity (New Advent)