6 “And they shall never perish:” you may hear the undertone, as if He had said to them, You shall perish for ever, because you are not of my sheep. “No one shall pluck them out of my hand.” Give still greater heed to this: “That which my Father gave me is greater than all.” What can the wolf do? What can the thief and the robber? They destroy none but those predestined to destruction. But of those sheep of which the apostle says, “The Lord knows them that are His;” and “Whom He did foreknow, them He also did predestinate; and whom He did predestinate, them He also called; and whom He called, them He also justified; and whom He justified, them He also glorified;” — there is none of such sheep as these that the wolf seizes, or the thief steals, or the robber slays.
He, who knows what He gave for them, is sure of their number. And it is this that He says: “No one shall pluck them out of my hand;” and in reference also to the Father, “That which my Father gave me is greater than all.” What did the Father give to the Son that was greater than all? To be His own only-begotten Son. What, then, means “gave”? Was He to whom He gave previously existent, or gave He in the act of begetting? For if He previously existed to whom He gave the gift of Sonship, there was a time when He was, and was not the Son.
Far be it from us to suppose that the Lord Christ ever was, and yet was not the Son. Of us such a thing may be said: there was a time when we were the sons of men, but were not the sons of God. For we are made the sons of God by grace, but He by nature, for such was He born. And yet not so, as that one may say, He did not exist till He was born; for He, who was coeternal with the Father, was never unborn. Let him who is wise understand: and whoever understands not, let him believe and be nourished, and he will come to understanding.
The Word of God was always with the Father, and always the Word; and because the Word, therefore the Son. So then, always the Son, and always equal. For it is not by growth but by birth that He is equal, who was always born, the Son of the Father, God of God, coeternal of the Eternal. But the Father is not God of the Son: the Son is God of the Father; therefore in begetting the Son, the Father “gave” Him to be God, in begetting He gave Him to be coeternal with Himself, in begetting He gave Him to be His equal.
This is that which is greater than all. How is the Son the life, and the possessor of life? What He has, He is: as for you, you are one thing, you have another. For example, you have wisdom, but are you wisdom itself? In short, because you yourself art not that which you have, should you lose what you have, you return to the state of no longer having it: and sometimes you re-acquire, sometimes you lose. As our eye has no light inherently in itself, it opens, and admits it; it shuts, and loses it.
It is not thus that the Son of God is God— not thus that He is the Word of the Father; and not thus is He the Word, that passes away with the sound, but that which abides in its birth. In such a way has He wisdom that He is Himself wisdom, and makes men wise: and life, that He is Himself the life, and makes others alive. This is that which is greater than all. The evangelist John himself looked to heaven and earth when wishing to speak of the Son of God; he looked, and rose above them all.
He thought on the thousands of angelic armies above the heavens; he thought, and, like the eagle soaring beyond the clouds, his mind overpassed the whole creation: he rose beyond all that was great, and arrived at that which was greater than all; and said, “In the beginning was the Word.” But because He, of whom is the Word, is not of the Word, and the Word is of Him, whose Word He is; therefore He says, “That which the Father gave me,” namely, to be His Word, His only-begotten Son, the brightness of His light, “is greater than all.” Therefore, “No one,” He says, “plucks my sheep out of my hand. No one can pluck them out of my Father's hand.”
Source: Tractates on the Gospel of John (New Advent)