I am curious to understand how others understand this passage.
What exactly is this “oneness” that is being prayed for by Jesus?
+Jn 17,20. “Neither pray I for these alone, but for them also who shall believe.”5
[2.] For since He was dying for them, and said, that “For their sakes I sanctify Myself,” lest any one should think that He did this for the Apostles only, He added, “Neither pray I for these only, but for them also who believe on Me through their word.” By this again He revived their souls, showing that the disciples should be many. For because He made common what they possessed peculiarly, He comforteth them by showing that they were being made the cause of the salvation of others.
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 ended6 in it. For at the beginning He saith, “A new commandment I give unto you” (c. 13,34); and here,
+Jn 17,21. “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.” (Luke vi. 36).
But what is, “In Us”?7 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 evenforetold 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?
+Jn 17,22. “And the glory which Thou gavest Me, I have given them.”8
That by miracles, that by doctrines,9 and, that they should be of one soul; for this is glory, that they should be one, and greater even than miracles. As men 10 admire God because there is no strife or discord in That Nature, and this is His greatest glory, “so too let these,” He saith, “from this cause become glorious.” “And how,” saith some one, “doth He ask the Father to give this to them, when He sixth that He Himself giveth it?” Whether His discourse be concerning miracles, or unanimity, or peace, He is seen Himself to have given these things to them; whence it is clear that the petition is made for the sake of their comfort.
+Jn 17,23. “I in them, and Thou in Me.” “How gave He the glory?” By being in them, and having the Father with Him, so as to weld them 11 together. But in another place He speaketh not so; He saith not that the Father cometh by Him, but, “that He and the Father come, and take up their abode with him,” 12 “there” removing the suspicion of Sabellius, “here” that of Arius. 13
“That they may be made perfect in one, and that the world may know that Thou hast sent Me.” (c. 14,23).
(He saith these latter words immediately after the other, to show that peace hath more power to attract men than a miracle; for as it is the nature of strife 14 to separate, so it is that of agreement to weld together.
“And I have 15 loved them as Thou hast loved Me.”
Here again the “as” means, as far as it is possible for a man to be loved; and the sure proof of His love is His giving Himself for them. After having told them that they shall be in safety, that they shall not be overturned, that they shall be holy, that many shall believe through them, that they shall enjoy great glory, that not He alone loved them, but the Father also; He next telleth them of what shah be after their sojourning here, 16 concerning the prizes and crowns laid up for them. (Chrysostom on John 82)
I have always thought that the way Chrysostom had articulated it was quite compelling concerning the “oneness” of Christs’ apostles. Although, I would tend to agree with Augustine on verse 22, he understands it as “the glory of heaven”, which he had given, prepared, and designed to give them in heaven. This seems to be the sense by the 24th verse.