João 17
Ambrose of Milan · Comentário Patrístico (catena)
João 17:19
Let us pay attention to the distinction of the Godhead from the flesh. In each there speaks one and the same Son of God, for each nature is present in him. And yet, while it is the same person who speaks, he does not always speak in the same way. At one time you see in him the glory of God, at another time human characteristics. As God he speaks the things of God because he is the Word. As man he speaks in a human way because he speaks in my nature.… Even the letter itself [here] teaches us that it is not the Godhead but the flesh that needed sanctification. For the Lord himself said, "And I sanctify myself for them," in order that you may acknowledge that he is both sanctified in the flesh for us and sanctifies by virtue of his divinity. — Exposition of the Christian Faith 2.9.77-78
João 17:21
No separation, then, is to be made of the Word from God the Father, no separation in power, no separation in wisdom because of the unity of the divine substance. Again, God the Father is in the Son, as we often times find it written, yet not as sanctifying one who lacks sanctification or as filling a void, for the power of God knows no void. Nor, again, is the power of the one increased by the power of the other, for there are not two powers but one power. Nor does Godhead entertain Godhead, for there are not two Godheads but one Godhead. We … shall be one in Christ through power received [from another] and dwelling in us. The letter [of the unity] is common, but the substance of God and the substance of humanity are different. We shall be one. The Father and the Son [already] are one. We shall be one by grace; the Son is so by substance. Again, unity by conjunction is one thing, unity by nature another. Finally, observe what it is that Scripture has already recorded: "That they may all be one, as you, Father, are in me, and I in you." Now notice that he did not say, "You in us, and we in you" but "You in me, and I in you." In this way he sets himself apart from his creatures. Further, he added "that they also may be in us," in order to separate here his dignity and his Father's dignity from us so that our union in the Father and the Son may appear the result not of nature but of grace, while the unity of the Father and the Son is the Son's, not by grace but by natural right of sonship. — Exposition of the Christian Faith 4.3.36-38
João 17:23
There are some … who in their desire to deny the unity of the divine substance try to make light of the love of the Father and the Son, because it is written, "You have loved them, as you have loved me." But when they say this, what else do they do but adopt a likeness of comparison between the Son of God and human beings? Can we indeed be loved by God as the Son is, in whom the Father is well-pleased? The Son is well-pleasing in himself; we are well-pleasing through him. For those in whom God sees his own Son after his own likeness, he admits through his Son into the favor of sons [children]. As we go through likeness unto likeness, in the same way, we are called to adoption through the generation of the Son. The eternal love of God's nature is one thing; that of grace is another. And if they start a debate on the words that are written, "And you have loved them, as you have loved me," and think a comparison is intended, they must think that the following also was said by way of comparison, "Be merciful, as your Father who is in heaven is merciful." It is also elsewhere said, "Be perfect, as my Father who is in heaven is perfect." But if he is perfect in the fullness of his glory, we are but perfect according to the growth of virtue within us. The Son also is loved by the Father according to the fullness of a love that always remains in him, but in us growth in grace merits the love of God. You see, then, how God has given grace to humankind, and do you want to separate the natural and indivisible love of the Father and the Son? And do you still strive to make nothing of words, where you note the mention of a unity of majesty? — Exposition of the Christian Faith 5.7.88-91