14 And first I ask what new element, destructive of His Godhead, can have been imported by birth into the nature of the Son? Universal reason rejects the supposition that a being can become different in nature, by the process of birth, from the being to which its birth is due; although we recognise the possibility that from parents, different in kind, an offspring sharing the nature of both, yet diverse from either, may be propagated. The fact is familiar in the case of beasts, both tame and wild.
But even in this case there is no real novelty; the new qualities already exist, concealed in the two different parental natures, and are only developed by the connection. The birth of their joint offspring is not the cause of that offspring's difference from its parents. The difference is a gift from them of various diversities, which are received and combined in one frame. When this is the case as to the transmission and reception even of bodily differences, is it not a form of madness to assert that the birth of God the Only-begotten was the birth from God of a nature inferior to Himself?
For the giving of birth is a function of the true nature of the transmitter of life; and without the presence and action of that true nature there can be no birth. The object of all this heat and passion is to prove that there was no birth, but a creation, of the Son of God; that the Divine nature is not His origin and that He does not possess that nature in His personal subsistence, but draws, from what was non-existent, a nature different in kind from the Divine. They are angry because He says, That which is born of the flesh is flesh, and that which is born of the Spirit is Spirit. For, since God is a Spirit, it is clear that in One born from Him there can be nothing alien or different from that Spirit from which He was born.
Thus the birth of God constitutes Him perfect God. And hence also it is clear that we must not say that He began to exist, but only that He was born. For there is a sense in which beginning is different from birth. A thing which begins to exist either comes into existence out of nothing, or developes out of one state into another, ceasing to be what it was before; so, for instance, gold is formed out of earth, solids melt into liquids, cold changes to warmth, white to red, water breeds moving creatures, lifeless objects turn into living.
In contrast to all this, the Son of God did not begin, out of nothing, to be God, but was born as God; nor had He an existence of another kind before the Divine. Thus He Who was born to be God had neither a beginning of His Godhead, nor yet a development up to it. His birth retained for Him that nature out of which He came into being; the Son of God, in His distinct existence, is what God is, and is nothing else.
Source: On the Trinity (New Advent)