John 17:9

"I pray for them." "What sayest Thou?" "Dost Thou teach the Father, as though He were ignorant? Dost Thou speak to Him as to a man who knoweth not?" "What then meaneth this distinction?" Seest thou that the prayer is for nothing else than that they may understand the love which He hath towards them? For He who not only giveth what He hath of His own, but also calleth on Another to do the same, showeth greater love. What then is, "I pray for them"? "Not for all the world," He saith, but "for them whom Thou hast given Me." He continually putteth the "hast given," that they might learn that this seemeth good to the Father. Then, because He had said continually, "they are Thine," and, "Thou gavest them unto Me," to remove any evil suspicion, and lest any one should think that His authority was recent, and that He had but now received them, what saith He?

When the Lord was speaking to the Father of those whom He already had as disciples, He said this also among other things: "I pray for them. I pray not for the world, but for those whom Thou hast given me." By the world, He now wishes to be understood those who live according to the lust of the word, and stand not in the gracious lot of such as were to be chosen by Him out of the world. Accordingly it is not for the world, but for those whom the Father hath given Him, that He expresses Himself as praying: for by the very fact of their having already been given Him by the Father, they have ceased to belong to that world for which He refrains from praying. And then He adds, "For they are Thine." For the Father did not lose those whom He gave, in the act of giving them to the Son; since the Son still goes on to say, "And all mine are Thine, and Thine are mine." Where it is sufficiently apparent how it is that all that belongs to the Father belongs also to the Son; in this way, namely, that He Himself is also God, and, of the Father born, is the Father's equal: and not as was said to one of the two sons, to wit, the elder, "Thou art ever with me; and all that I have is thine." For that was said of all those creatures which are inferior to the holy rational creature, and are certainly subordinate to the Church; wherein its universal character is understood as including those two sons, the elder and the younger, along with all the holy angels, whose equals we shall be in the kingdom of Christ and of God: but here it was said, "And all mine are Thine, and Thine are mine," with this meaning, that even the rational creature is itself included, which is subject only to God, so that all beneath it are also subject to Him. As it then belongs to God the Father, it would not at the same time be the Son's likewise, were He not equal to the Father: for to it He was referring when He said, "I pray not for the world, but for those whom Thou hast given me: for they are Thine, and all mine are Thine, and Thine are mine." Nor is it morally admissible that the saints, of whom He so spake, should belong to any save to Him by whom they were created and sanctified: and for the same reason, everything also that is theirs must of necessity be His also to whom they themselves belong, Accordingly, since they belong both to the Father and to the Son, they demonstrate the equality of those to whom they equally belong.

(Tr. cvi) When He adds, I pray not for the world, by the world He means those who live according to the lust of the world, and have not the lot to be chosen by grace out of the world, as those had for whom He prayed: But for them which Thou hast given Me. It was because the Father had given Him them, that they did not belong to the world. Nor yet had the Father, in giving them to the Son, lost what He had given: For they are Thine.