John 17:21 ἵνα πάντες ἓν ὦσιν, καθὼς σύ, πάτερ, ἐν ἐμοὶ κἀγὼ ἐν σοί, ἵνα καὶ αὐτοὶ ἐν ἡμῖν ὦσιν, ἵνα ὁ κόσμος πιστεύῃ ὅτι σύ με ἀπέστειλας. 22 κἀγὼ τὴν δόξαν ἣν δέδωκάς μοι δέδωκα αὐτοῖς, ἵνα ὦσιν ἓν καθὼς ἡμεῖς ἕν· 23 ἐγὼ ἐν αὐτοῖς καὶ σὺ ἐν ἐμοί, ἵνα ὦσιν τετελειωμένοι εἰς ἕν, ἵνα γινώσκῃ ὁ κόσμος ὅτι σύ με ἀπέστειλας καὶ ἠγάπησας αὐτοὺς καθὼς ἐμὲ ἠγάπησας.21 that they may all be one; even as You, Father, are in Me and I in You, that they also may be in Us, so that the world may believe that You sent Me. 22 "The glory which You have given Me I have given to them, that they may be one, just as We are one; 23 I in them and You in Me, that they may be perfected in unity, so that the world may know that You sent Me, and loved them, even as You have loved Me. (NASB)
Jesus begins his petition for unity in John 17:21 -- "That they all may be one" … how? … "just as you, Father, are in me and I in you.
Perichoretic union, (= mutual indwelling of persons), is the focus of the oneness that Christ prays for.
How can we be “perichoretically” one? The rest of the verse explains: “in order that they also might be in us.” We are “one” as Jesus prayed, when we are united with Father and Son.
Verse 22 continues the idea: “I have given them the glory that you gave me in order that they may be one as we are one.”
What is the “glory” that Jesus gave His disciples and how does that result in their unity?
My first stab at defining the “glory” would be the privilege of union with Christ. That is the next thing Jesus says: “I in them and you in me, in order that they might be perfected in oneness.”
1. This is perhaps the passage most abused by evangelicals in a plea for visible, corporate unity. The unity for which Jesus was praying was not the unity of believer with believer, but rather of all believers with Christ and the Father. This has implications for how we relate to others, but I do not believe it implies visible, denominational, or structural unity.
2. What an incredible prayer! Jesus wants us in Him. He is in the Father (v22), so that puts us in the Father as well. The Father is in Him (22) and He is in us (23), so that means the Father is in us too.
3. Jesus opens the door to the mysterious ‘oneness’ of God our Triune God. Jesus and the Father are “one” through mutual indwelling. We become “one” in the same way: we indwell God and He indwells us. Here is marvelous truth, profoundest mystery.
4. Yet, the mystery of mutual indwelling is ours through union with Christ through faith. This participation in divine perichoresis affords us the privilege of fellowship with the Father and the Son (1 John 1:4), and grants us the fellowship of the Holy Spirit (Phil. 2:2).
Surely, the appropriate response is glad praise, enthusiastic participation in triune fellowship, adoring humility and submission.
I marvel at the privilege that union with Christ has afforded me! Grant me grace to delight more fully and participate more sensibly in the oneness that is already mine in Christ by the Spirit.
On the grounds of Jesus’ name, I pray.
 So also Barnes, Jamieson-Faussett-Brown; Gill: “the Gospel is meant, which is glorious in its author, matter and subject, in its doctrines, in the blessing: grace it reveals, and promises it contains, and in the efficacy and usefulness of it to the souls of men. This was given to Christ, and he gave it to his disciples.” Wesley: “The glory of the only begotten shines in all the sons of God. How great is the majesty of Christians.” Clarke: “the glorious privilege of becoming sons of God; that, being all adopted children of the same Father, … however, … the words may therefore be understood of the glory which they were to share with him in heaven.