37 That true and absolute and perfect doctrine, which forms our faith, is the confession of God from God and God in God, by no bodily process but by Divine power, by no transfusion from nature into nature but through the secret and mighty working of the One nature; God from God, not by division or extension or emanation, but by the operation of a nature which brings into existence, by means of birth, a nature One with itself. The facts shall receive a fuller treatment in the next book, which is to be devoted to an exposition of the teaching of the Evangelists and Apostles; for the present we must maintain our assertion and belief by means of the Law and the Prophets.
The nature with which God is born is necessarily the same as that of His Source. He cannot come into existence as other than God, since His origin is from none other than God. His nature is the same, not in the sense that the Begetter also was begotten— for then the Unbegotten, having been begotten, would not be Himself— but that the substance of the Begotten consists in all those elements which are summed up in the substance of the Begetter, Who is His only Origin. Thus it is due to no external cause that His origin is from the One, and that His existence partakes the Unity; there is no novel element in Him, because His life is from the Living; no element absent, because the Living begot Him to partake His own life.
Hence, in the generation of the Son, the incorporeal and unchangeable God begets, in accordance with His own nature, God incorporeal and unchangeable; and this perfect birth of incorporeal and unchangeable God from incorporeal and unchangeable God involves, as we see in the light of the revelation of God from God, no diminution of the Begetter's substance. And so God the Only-begotten bears witness through the holy Moses; See, see that I am God, and there is no God beside Me.
For there is no second Divine nature, and so there can be no God beside Him, since He is God, yet by the powers of His nature God is also in Him. And because He is God and God is in Him, there is no God beside Him; for God, than Whom there is no other Source of Deity, is in Him, and consequently there is within Him not only His own existence, but the Author of that existence.
Source: On the Trinity (New Advent)