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.
38. This saving faith which we profess is sustained by the spirit of prophecy, speaking with one voice through many mouths, and never, through long and changing ages, bearing an uncertain witness to the truths of revelation. For instance, the words which, as we are told through Moses, were spoken by God the Only-begotten, are confirmed for our better instruction by the prophetic spirit, speaking this time through those men of stature — For God is in You, and there is no God beside You. You are God, and we knew it not, O God of Israel, the Saviour. Let heresy fling itself with its utmost effort of despair and rage against this declaration of a name and nature inseparably joined, and rend in two, if its furious struggles can, a union perfect in title and in fact. God is in God and beside Him there is no God. Let heresy, if it can, divide the God within from the God within Whom He is, and classify, Each after His kind, the members of that mystic union. For when He says God is in You, He teaches that the true nature of God the Father is present in God the Son; for we must understand that it is the God Who is Exodus 3:14 that is in Him. And when He adds, And there is no God beside You, He shows that outside Him there is no God, since God's dwelling is within Himself. And the third assertion, You are God and we knew it not, sets forth for our instruction what must be the confession of the devout and believing soul. When it has learned the mysteries of the Divine birth, and the name Emmanuel which the angel announced to Joseph, it must cry, You are God, and we knew it not, O God of Israel, the Saviour. It must recognise the subsistence of the Divine nature in Him, inasmuch as God is in God, and the nonexistence of any other God except the true. For, He being God and God being in Him, the delusion of another God, of whatever kind, must be surrendered. Such is the message of the prophet Isaiah; he bears witness to the indivisible and inseparable Godhead of Father and of Son.
39. Jeremiah also, a prophet equally inspired, has taught that God the Only-begotten is of a nature one with that of God the Father. His words are:— This is our God, and there shall be none other likened unto Him, Who has found out all the way of knowledge, and has given it unto Jacob His servant, and to Israel His beloved. Afterward He was seen upon earth, and dwelt among men. Why try to transform the Son of God into a second God? Learn to recognise and to confess the One True God. No second God is likened to Christ, and so can claim to be God. He is God from God by nature and by birth, for the Source of His Godhead is God. And, again, He is not a second God, for no other is likened unto Him; the truth that is in Him is nothing else than the truth of God. Why link together, in pretended devotion to the unity of God, true and false, base and genuine, unlike and unlike? The Father is God and the Son is God. God is in God; beside Him there is no God, and none other is likened unto Him so as to be God. If in these Two you shall recognise the Unity, instead of the solitude, of God, you will share the Church's faith, which confesses the Father in the Son. But if, in ignorance of the heavenly mystery, you insist that God is One in order to enforce the doctrine of His isolation, then you are a stranger to the knowledge of God, for you deny that God is in God.