John 17:21

And the glory which Thou hast given Me I have given them; that they may be one, as We are one: I in them, and Thou in Me, that they may be made perfect in one."

But let these things which were done by Stephen be passed by for the present, lest, while we remember his audacity and pride, we bring a more lasting sadness on ourselves from the things that he has wickedly done. And knowing, concerning you, that you have settled this matter, concerning which there is now a question, according to the rule of truth and the wisdom of Christ; we have exulted with great joy, and have given God thanks that we have found in brethren placed at such a distance such a unanimity of faith and truth with us. For the grace of God is mighty to associate and join together in the bond of charity and unity even those things which seem to be divided by a considerable space of earth, according to the way in which of old also the divine power associated in the bond of unanimity Ezekiel and Daniel, though later in their age, and separated from them by a long space of time, to Job and Noah, who were among the first; so that although they were separated by long periods, yet by divine inspiration they felt the same truths. And this also we now observe in you, that you who are separated from us by the most extensive regions, approve yourselves to be, nevertheless, joined with us in mind and spirit. All which arises from the divine unity. For even as the Lord who dwells in us is one and the same, He everywhere joins and couples His own people in the bond of unity, whence their sound has gone out into the whole earth, who are sent by the Lord swiftly running in the spirit of unity; as, on the other hand, it is of no advantage that some are very near and joined together bodily, if in spirit and mind they differ, since souls cannot at all be united which divide themselves from God's unity. "For, lo," it says, "they that are far from Thee shall perish." But such shall undergo the judgment of God according to their desert, as depart from His words who prays to the Father for unity, and says, "Father, grant that, as Thou and I are one, so they also may be one in us."

(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.

[This passage] shows that since human beings cannot, so to speak, be fused back into God or themselves coalesce into one undistinguished mass, this oneness must arise from unity of will, as all perform actions pleasing to God and unite with one another in the harmonious agreement of their thoughts. Therefore, it is not nature that makes them one but will.

The Saviour, then, saying of us, "As Thou, Father, art in Me, and I in Thee, that they too may be one in Us," does not signify that we were to have identity with Him; for this was shewn from the instance of Jonah; but it is a request to the Father, as John has written, that the Spirit should be vouchsafed through Him to those who believe, through whom we are found to be in God, and in this respect to be conjoined in Him. For since the Word is in the Father, and the Spirit is given from the Word, He wills that we should receive the Spirit, that, when we receive It, thus having the Spirit of the Word which is in the Father, we too may be found on account of the Spirit to become One in the Word, and through Him in the Father.

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.

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?

(Hom. lxxxii) Another ground of consolation to them, that they were to be the cause of the salvation of others.

(Hom. lxxxii) And with this prayer for unanimity, He concludes His prayer; and then begins a discourse on the same subject: A new commandment I give unto you, that ye love one another.

(Hom. lxxxii) This as again does not express perfect likeness, but only likeness as far as it was possible in men; as when He saith, Be ye merciful, even as your Father, which is in heaven, is merciful. (Luke 6:36)

(Hom. lxxxii) For there is no scandal so great as division, whereas unity amongst believers is a great argument for believing; as He said at the beginning of His discourse, By this shall all men know that ye are My disciples, if ye have love one to another. For if they quarrel, they will not be looked on as the disciples of a peacemaking Master. And I, He saith, not being a peacemaker, they will not acknowledge Me as sent from God.

(Hom. lxxxii. 2) By glory, He means miracles, and doctrines, and unity; which latter is the greater glory. For all who believed through the Apostles are one. If any separated, it was owing to men's own carelessness; not but that our Lord anticipates this happening.

(Hom. lxxxii) 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 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."

“As we are Father, Son and Holy Spirit, one God,” [Jesus might say], “so may they be one people in themselves, that is, like dear children and partakers of the divine nature.” Call the church what you will—bride, sister, mother—its assembly is but one and never lacks husband, brother or son. [The church’s] faith is one, and it is not defiled by variety of doctrine or divided by heresies. [The church] continues a virgin. Wherever the Lamb goes, it follows him. It alone knows the song of Christ.

(Tr. cix) When our Lord had prayed for His disciples, whom He named also Apostles, He added a prayer for all others who should believe on Him; Neither pray I for these alone, but for all others who shall believe on Me through their word.

(Tr. cix) All, i. e. not only those who were then alive, but those who were to be born; not those only who heard the Apostles themselves, but us who were born long after their death. We have all believed in Christ through their word: for they first heard that word from Christ, and then preached it to others, and so it has come down, and will go down to all posterity. We may see that in this prayer there are some disciples whom He does not pray for; for those, i. e. who were neither with Him at the time, nor were about to believe on Him afterwards through the Apostles' word, but believed already. Was Nathanael with Him then, or Joseph of Arimathea, and many others, who, John says, believed on Him? I do not mention old Simeon, or Anna the prophetess, Zacharias, Elisabeth, or John the Baptist; for it might be answered that it was not necessary to pray for dead persons, such as these who departed with such rich merits. With respect to the former then we must understand that they did not yet believe in Him, as He wished, but that after His resurrection, when the Apostles were taught and strengthened by the Holy Spirit, they attained to a right faith. The case of Paul however still remains, An Apostle not of men, or by men; (Gal. 1:1) and that of the robber, who believed when even the teachers themselves of the faith fell away. We must understand then, their word, to mean the word of faith itself which they preached to the world; it being called their word, because it was preached in the first instance and principally by them; for it was being preached by them, when Paul received it by revelation from Jesus Christ Himself. And in this sense the robber too believed their word. Wherefore in this prayer the Redeemer prays for all whom He redeemed, both present and to come. And then follows the thing itself which He prays for, That they all may be one. He asks that for all, which he asked above for the disciples; that all both we and they may be one.

(Tr. cx) We must particularly observe here, that our Lord did not say, that we may be all one, but that they may be all one, as Thou, Father, in Me, and I in Thee, are one, understood. For the Father is so in the Son, that They are one, because They are of one substance; but we can be one in Them, but not with Them; because we and They are not of one substance. They are in us, and we in Them, so as that They are one in Their nature, we one in ours. They are in us, as God is in the temple; we in Them, as the creature is in its Creator. Wherefore He adds, in Us, to show, that our being made one by charity, is to be attributed to the grace of God, not to ourselves.

(iv. de. Trin. c. ix) Or that in ourselves we cannot be one, severed from each other by diverse pleasures, and lusts, and the pollution of sin, from which we must be cleansed by a Mediator, in order to be one in Him.

(Tr. cx) But why does He say, That the world may believe that Thou hast sent Me? Will the world believe when we shall all be one in the Father and the Son? Is not this unity that peace eternal, which is the reward of faith, rather than faith itself? For though in this life all of us who hold in the same common faith are one, yet even this unity is not a means to belief, but the consequence of it. What means then, That all may be one, that the world may believe? He prays for the world when He says, Neither pray I for these alone, but for all those who shall believe on Me through their word. Whereby it appears that He does not make this unity the cause of the world believing, but prays that the world may believe, as He prays that they all may be one. The meaning will be clearer if we always put in the word ask; I ask that they all may be one; I ask that they may be one in Us; I ask that the world may believe that Thou hast sent Me.

(Tr. cx) Then our Saviour, Who, by praying to the Father, showed Himself to be man, now shows that, being God with the Father, He doth what He prays for: And the glory which Thou gavest Me, I have given them. What glory, but immortality, which human nature was about to receive in Him? For that which was to be by unchangeable predestination, though future, He expresses by the past tense. That glory of immortality, which He says was given Him by the Father, we must understand He gave Himself also. For when the Son is silent of His own cooperation in the Father's work, He shows His humility: when He is silent of the Father's cooperation in His work, He shows His equality. In this way here He neither disconnects Himself with the Father's work, when He says, The glory which Thou gavest Me, nor the Father with His work, when He says, I have given them. But as He was pleased by prayer to the Father to obtain that all might be one, so now He is pleased to effect the same by His own gift; for He continues, That all may be one, even as We are one.

(Tr. cx. 4) Nor is this said, however, as if to mean that the Father was not in us, or we in the Father. He only means to say, that He is Mediator between God and man. And what He adds, That they may be made perfect in one, shows that the reconciliation made by this Mediator, was carried on even to the enjoyment of everlasting blessedness. So what follows, That the world may know that Thou hast sent Me, must not be taken to mean the same as the words just above, That the world may believe. For as long as we believe what we do not see, we are not yet made perfect, as we shall be when we have merited to see what we believe. So that when He speaks of their being made perfect, we are to understand such a knowledge as shall be by sight, not such as is by faith. These that believe are the world, not a permanent enemy, but changed from an enemy to a friend; as it follows: And hast loved them, as Thou hast loved Me. The Father loves us in the Son, because He elected us in Him. These words do not prove that we are equal to the Only Begotten Son; for this mode of expression, as one thing so another, does not always signify equality. It sometimes only means, because one thing, therefore another. And this is its meaning here: Thou hast loved them, as Thou hast loved Me, i. e. Thou hast loved them, because Thou hast loved Me. There is no reason for God loving His members, but that He loves him. But since He hateth nothing that He hath made, who can adequately express how much He loves the members of His Only Begotten Son, and still more the Only Begotten Himself.

How, then, are we to understand Him when He says, "That they also may be one in us, that the world may believe that Thou hast sent me," but just in this way, that He did not assign the cause of the world believing to the fact that those others are one, as if it believed on the ground that it saw them to be one; for the world itself here consisteth of all who by their own believing become one; but in His prayer He said, "That the world may believe," just as in His prayer He also said, "That they all may be one;" and still further in the same prayer, "That they also may be one in us." For the words, "they all may be one," are equivalent to "the world may believe," since it is by believing that they become one, perfectly one; that is, those who, although one by nature, had ceased to be so by their mutual dissensions. In fine, if the verb which He uses, "I pray," be understood in the third clause, or rather, to make the whole fuller, be everywhere supplied, the explanation of this sentence will be all the clearer: I pray "that they all may be one; as Thou, Father, in me, and I in Thee;" I pray "that they also may be one in us;" I pray "that the world may believe that Thou hast sent me." And, mark, He added the words "in us" in order that we may know that our being made one in that love of unchanging faithfulness is to be attributed to the grace of God, and not to ourselves: just as the apostle, after saying, "For ye were at one time darkness, but now are ye light," that none might attribute the doing of this to themselves, added, "in the Lord."

But then after saying, "That they also may be one in us," He added, "That the world may believe that Thou hast sent me." What does He mean by this? Is it that the world will then be brought to the faith, when we shall all be one in the Father and Son? Is not such a state the everlasting peace, and the reward of faith, rather than faith itself? For we shall be one not in order to our believing, but because we have believed. But although in this life, because of the common faith itself, all who believe in one are one according to the words of the apostle, "For ye are all one in Christ Jesus;" even thus we are one, not in order to our believing, but because we do believe. What, then, is meant by the words, "That they all may be one, that the world may believe"? This, doubtless, that the "all" are themselves the believing world. For those who shall be one are not of one class, and the world that is thereafter to believe on this very ground that these shall be one, of another; since it is perfectly certain that He says, "That they all may be one," of those of whom He had said before, "Neither pray I for these alone, but for those also who shall believe on me through their word," immediately adding as He does, "That they all may be one." And this "all," what is it but the world; not certainly that which is hostile, but that which is believing? For you see here that He who had said, "I pray not for the world," now prayeth for the world that it may believe. For there is a world whereof it is written, "That we might not be condemned with this world." For that world He prayeth not, for He is fully aware to what it is predestinated. And there is a world whereof it is written, "For the Son of man came not to condemn the world, but that the world through Him might be saved;" and hence the apostle also says, "God was in Christ, reconciling the world unto Himself." For this world it is that He prayeth, in saying, "That the world may believe that Thou hast sent me." For through this faith the world is reconciled unto God when it believes in the Christ whom God has sent.

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.

Our Lord Jesus Christ did not pray for the twelve apostles alone. He prayed for all who were destined in every age to yield to and obey the words that call them to be holy by believing and to be purified through participation in the Holy Spirit.… “May they all be one,” he prayed. “As you Father are in me and I am in you, may they also be one in us.” … The only Son shines out from the very substance of the Father and possesses the Father completely in his own nature. He became man, according to the Scriptures, blending himself, so to speak, with our nature by an inexplicable union with an earthly body.… In himself he somehow united totally disparate natures to make us sharers in the divine nature.The communion and abiding presence of the Spirit has passed even to ourselves. This was experienced first through Christ and in Christ when he was seen to have become like us, that is, a human being anointed and sanctified. By nature, however, he was God, for he proceeded from the Father. It was with his own Spirit that he sanctified the temple of his body and also, in a way befitting it, the world of his creation. Through the mystery of Christ, then, sharing in the Holy Spirit and union with God has become possible also for us, for we are all sanctified in him.
By his own wisdom and the Father’s counsel he devised a way of bringing us all together and blending us into a unity with God and one another, even though the differences between us give us each in both body and soul a separate identity. For in Holy Communion he blesses with one body, which is his own, those who believe in him, and he makes them one body with himself and one another. Who could separate those who are united to Christ through that one sacred body or destroy their true union with one another? If “we all share one loaf,” we all become one body, for Christ cannot be divided.
So it is that the church is the body of Christ, and we are its members. For since we are all united to Christ through his sacred body, having received that one indivisible body into our own, our members are not our own but his.

[Responding to the question "Many people are quite unable to understand the theological differences which have caused divisions in the Christian Church. Do you consider that these differences are fundamental, and is the time now ripe for reunion?"]

The time is always ripe for reunion. Divisions between Christians are a sin and a scandal, and Christians ought at all times to be making contributions toward reunion, if it is only by their prayers. I am only a layman and a recent Christian, and I do not know much about these things, but in all the things which I have written and thought I have always stuck to traditional, dogmatic positions. The result is that letters of agreement reach me from what are ordinarily regarded as the most different kinds of Christians; for instance, I get letters from Jesuits, monks, nuns, and also from Quakers and Welsh Dissenters, and so on. So it seems to me that the "extremist" elements in every church are nearest one another and the liberal and "broad-minded" people in each body could never be united at all. The world of dogmatic Christianity is a place in which thousands of people of quite different types keep on saying the same thing, and the world of "broad-mindedness" and watered-down "religion" is a world where a small number of people (all of the same type) say totally different things and change their minds every few minutes. We shall never get reunion from them.