13 And again, what a wonderful example of heretical ingenuity is this:— “Nor as if He were previously in being, and afterwards born or created afresh, to be a Son.” God, since He was born from God, was assuredly not born from nothing, nor from things non-existent. His birth was that of the eternally living nature. Yet, though He is God, He is not identical with the pre-existing God; God was born from God Who existed before Him; in, and by, His birth He partook of the nature of His Source.
If we are speaking words of our own, all this is mere irreverence; but if, as we shall prove, God Himself has taught us how to speak, then the necessity is laid upon us of confessing the Divine birth in the sense revealed by God. And it is this unity of nature in Father and in Son, this ineffable mystery of the living birth, which the madness of heresy is struggling to banish from belief, when it says, “Nor as if He were previously in being, and afterwards born, or created afresh, to be a Son.”
Now who is senseless enough to suppose that the Father ceased to be Himself; that the same Person Who had previously existed was afterwards born, or created afresh, to be the Son? That God disappeared, and that His disappearance was followed by an emergence in birth, when, in fact, that birth is evidence of the continuous existence of its Author? Or who is so insane as to suppose that a Son can come into existence otherwise than through birth? Who so void of reason as to say that the birth of God resulted in anything else than in God being born?
The abiding God was not born, but God was born from the abiding God; the nature bestowed in that birth was the very nature of the Begetter. And God by His birth, which was from God into God, received, because His was a true birth, not things new-created but things which were and are the permanent possession of God. Thus it is not the pre-existent God that was born; yet God was born, and began to exist, out of and with the properties of God. And thus we see how heresy, throughout this long prelude, has been treacherously leading up to this most blasphemous doctrine.
Its object being to deny God the Only-begotten, it starts with what purports to be a defence of truth, to go on to the assertion that Christ is born not from God but out of nothing, and that His birth is due to the Divine counsel of creation from the non-existent.
Source: On the Trinity (New Advent)