2 And then He adds, “For they are Yours.” 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 Yours, and Yours 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, “You are ever with me; and all that I have is yours.” 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 Yours, and Yours 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 You have given me: for they are Yours, and all mine are Yours, and Yours are mine.” Nor is it morally admissible that the saints, of whom He so spoke, 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. But when He says, speaking of the Holy Ghost, “All things that the Father has are mine; therefore said I, that He shall take of mine, and shall show it unto you,” He referred to those things which concern the actual deity of the Father, and in which He is equal to Him, in having all that He has. And no more was it of the creature, which is subject to the Father and the Son, that the Holy Spirit was to receive that whereof He said, “He shall receive of mine;” but most certainly of the Father, from whom the Spirit proceeds, and of whom also the Son is born.
Source: Tractates on the Gospel of John (New Advent)