54 The mystery of that other timeless birth I will not yet touch upon: its treatment demands an ampler space than this. For the present I will speak of the Incarnation only. Tell me, I pray, you who pry into secrets of Heaven, the mystery of Christ born of a Virgin and His nature; whence will you explain that He was conceived and born of a Virgin? What was the physical cause of His origin according to your disputations? How was He formed within His mother's womb? Whence His body and His humanity?
And lastly, what does it mean that the Son of Man descended from heaven Who remained in heaven? It is not possible by the laws of bodies for the same object to remain and to descend: the one is the change of downward motion; the other the stillness of being at rest. The Infant wails but is in Heaven: the Boy grows but remains ever the immeasurable God. By what perception of human understanding can we comprehend that He ascended where He was before, and He descended Who remained in heaven?
The Lord says, What if you should behold the Son of Man ascending there where He was before ? The Son of Man ascends where He was before: can sense apprehend this? The Son of Man descends from heaven, Who is in heaven: can reason cope with this? The Word was made flesh: can words express this? The Word becomes flesh, that is, God becomes Man: the Man is in heaven: the God is from heaven. He ascends Who descended: but He descends and yet does not descend. He is as He ever was, yet He was not ever what He is. We pass in review the causes, but we cannot explain the manner: we perceive the manner, and we cannot understand the causes. Yet if we understand Christ Jesus even thus, we shall know Him: if we seek to understand Him further we shall not know Him at all.
Source: On the Trinity (New Advent)