John Cassian, On the Incarnation of the Lord, Against Nestorius

Therefore all grace, power, might, Divinity, aye, and the fulness of actual Divinity and glory have ever existed together with Him and in Him, whether in heaven or in earth or in the womb or at His birth. Nothing that is proper to God was ever wanting to God. For the Godhead was ever present with God, no where and at no time severed from Him. For everywhere God is present in His completeness and in His perfection. He suffers no division or change or diminution; for nothing can be either added to God or taken away from Him, for He is subject to no diminution of Divinity, as to no increase of It. He was the same Person then on earth who was also in heaven: the same Person in His low estate who was also in the highest: the same Person in the littleness of manhood as in the glory of the Godhead. And so the Apostle was right in speaking of the grace of Christ when He meant the grace of God. For Christ was everything that God is. At the very time of His conception as man there came all the power of God, all the fulness of the Godhead; for thence came all the perfection of the Godhead, whence was His origin. Nor was that Human nature of His57 Homo ille. ever without the Deity as it received from Deity the very fact of its existence. And so, to begin with, whether you like it or no, you cannot deny this; viz., that the Lord Jesus Christ is the Son of God, especially as the archangel declares in the gospels: “That holy thing which shall be born of thee shall be called the Son of God.” But when this is established then remember that whatever you read of Christ you read of the Son of God: whatever you read of the Lord or Jesus belongs to the Son of God. And so when you recognize a title of Divinity in all these terms which you hear uttered, as you see that in each case you ought to understand that the Son of God is meant, prove to me, if you like, how you can separate the Godhead from the Son of God.