14 And then again, after an interval designed to prepare us for what is coming, their heresy delivers this assault—“While the Son, put forth outside time, and created and established before the worlds, did not exist before He was born.” This “He did not exist before He was born” is a form of words by which the heresy flatters itself that it gains two ends; support for its blasphemy, and a screen for itself if its doctrine be arraigned. A support for its blasphemy, because, if He did not exist before He was born, He cannot be of one nature with His eternal Origin.
He must have His beginning out of nothing, if He have no powers but such as are coeval with His birth. And a screen for its heresy, for if this statement be condemned, it furnishes a ready answer. He that did exist, it will be said, could not be born; being in existence already, He could not possibly come into being by passing through the process of birth, for the very meaning of birth is the entry into existence of the being that is born. Fool and blasphemer! Who dreams of birth in the case of Him Who is the unborn and eternal?
How can we think of God, Who is, being born, when being born implies the process of birth? It is the birth of God the Only-begotten from God His Father that you are striving to disprove, and it was your purpose to escape the confession of that truth by means of this “He did not exist before He was born;” the confession that God, from Whom the Son of God was born, did exist eternally, and that it is from His abiding nature that God the Son draws His existence through birth. If, then, the Son is born from God, you must confess that His is a birth of that abiding nature; not a birth of the pre-existing God, but a birth of God from God the pre-existent.
Source: On the Trinity (New Advent)