John 17:21

(vii de Trin) And this unity is recommended by the great example of unity: As Thou, Father, art in Me, and I in Thee, that they also may be one in Us, i. e. that as the Father is in the Son, and the Son in the Father, so, after the likeness of this unity, all may be one in the Father and in the Son. (viii. de Trin) Heretics endeavouring to get over the words, I and My Father are one, as a proving unity of nature, and to reduce them to mean a unity simply of natural love, and agreement of will, bring forwards these words of our Lord's as an example of this kind of unity: That they may be all one, as Thou, Father, art in Me, and I in Thee. But though impiety can cheat its own understanding, it cannot alter the meaning of the words themselves. For they who are born again of a nature that gives unity in life eternal, they cease to be one in will merely, acquiring the same nature by their regeneration: but the Father and Son alone are properly one, because God, only-begotten of God, can only exist in that nature from which He is derived. (viii. de Trin) Or, the world will believe that the Son is sent from the Father, for that reason, viz. because all who believe in Him are one in the Father and the Son.

After having thus spoken concerning their salvation, and their being sanctified by faith and the Sacrifice, He afterwards speaketh of concord, and finally closeth his discourse with this, having begun with it and ended in it. For at the beginning He saith, "A new commandment I give unto you"; and here, "That they all may be one, as Thou, Father, art in Me and I in Thee." Here again the "as" doth not denote exact similarity in their case, (for it was not possible for them in so great a degree,) but only as far as was possible for men. Just as when He saith "Be ye merciful, as your Father."

But what is, "In Us"? In the faith which is on Us. Because nothing so offends all men as divisions, He provideth that they should be one. "What then," saith some one, "did He effect this?" Certainly He effected it. For all who believe through the Apostles are one, though some from among them were torn away. Nor did this escape His knowledge, He even foretold it, and showed that it proceeded from men's slack-mindedness. "That the world may believe that Thou hast sent Me." As He said in the beginning, "By this shall all men know that ye are My disciples, if ye love one another," And how should they hence believe? "Because," He saith, "Thou art a God of peace." If therefore they observe the same as those of whom they have learnt, their hearers shall know the teacher by the disciples, but if they quarrel, men shall deny that they are the disciples of a God of peace, and will not allow that I, not being peaceable, have been sent from Thee. Seest thou how, unto the end, He proveth His unanimity with the Father?

Elsewhere He says of Himself and the Father, We will come and make Our abode with Him; by the mention of two persons, stopping the mouths of the Sabellians. Here by saying that the Father comes to the disciples through Him, He refutes the notion of the Arians.

After the Lord Jesus had prayed for His disciples whom He had with Him at the time, and had conjoined with them others who were also His own, by saying, "Neither pray I for these alone, but for them also who shall believe on me through their word," as if we were inquiring what or wherefore He prayed for them, He straightway subjoined, "That they all may be one; as Thou, Father, [art] in me, and I in Thee, that they also may be one in us." And a little above, while still praying for the disciples alone who were then with Him, He said, "Holy Father, keep in Thine own name those whom Thou hast given me, that they may be one, as we are." It is the same thing, therefore, that He now also prayed for in our behalf, as He did at that time in theirs, namely, that all-to wit, both we and they-may be one. And here we must take particular notice that the Lord did not say that we all may be one, but, "that they all may be one; as Thou Father, in me, and I in Thee"; because He had also said before of the disciples who were with Him, "That they may be one, as we are." The Father, therefore, is in the Son, and the Son in the Father, in such a way as to be one, because they are of one substance; but while we may indeed be in them, we cannot be one with them; for they and we are not of one substance, in as far as the Son is God along with the Father. But in as far as He is man, He is of the same substance as we are. But at present He wished rather to call attention to that other statement which He made use of in another place, "I and the Father are one," where He intimated that His own nature was the same with that of the Father. And accordingly, though the Father and Son, or even the Holy Spirit, are in us, we must not suppose that they are of one nature with ourselves. And hence they are in us, or we are in them, in this sense, that they are one in their own nature, and we are one in ours. For they are in us, as God in His temple; but we are in them, as the creature in its Creator.