8 This again is a notable utterance of the Father concerning Him: From the womb, before the morning star I begot You. Here, as we have often said already, nothing derogatory to God is implied in the concession to our weakness of understanding; as though, because He said that He begot Him from the womb, He were therefore composed of inner and outer parts, which unite to form His members, and owed His being to the same causes within time to which earthly bodies owe theirs; when in fact He Whose existence is due to no natural necessities, free and perfect, and eternal Lord of all nature, in explanation of the true character of the birth of His Only-begotten, points to power of His own unchangeable nature.
For though Spirit be born of Spirit (consistently, be it remembered, with the true character of Spirit, through which itself is also Spirit), nevertheless its only cause for being born lies within those perfect and unchangeable causes. And though it is from a perfect and unchangeable cause that it is born, it must needs be born from that cause, in accordance with the true character of that cause. Now the necessary process of human birth is conditioned by the causes which operate upon the womb.
But as God is not made up of parts, but is unchangeable as being Spirit, for God is Spirit, He is subject to no natural necessity working within Him. But since He was telling us of the birth of Spirit from Spirit, He instructed our understanding by an example from causes which work among us: not to give an example of the manner of birth, but to declare the fact of generation; not that the example might prove Him subject to necessity, but that it might enlighten our mind. If, therefore, God Only-begotten is a created being, what meaning is there in a revelation which uses the common facts of human birth to indicate that He was divinely generated?
Source: On the Trinity (New Advent)