I pray also for those who will believe in me through their message, 21 that all of them may be one, Father, just as you are in me and I am in you. May they also be in us so that the world may believe that you have sent me. 22 I have given them the glory that you gave me, that they may be one as we are one— 23 I in them and you in me—so that they may be brought to complete unity.
As I have written in a previous post on this subject, a striking feature of this prayer is the extremely strong expression of the Trinity that's present in it. While the Holy Spirit isn't mentioned by name, it is through the work of the Spirit that The Church will continue to be nurtured and empowered following the ascension of Christ, so His role can strongly be inferred in the Son praying to the Father for The Church.
A very key word in this passage to me is the word "as". "May they be one as you are in me and I am in you", and "that they may be one as we are one." As Jesus conveys these sentiments he twice prays for the unity of the Church, and both times he uses "as" to signify that there is a certain way or manner that He desires for the Church to be one; namely in the same way that the Father and Son are unified and one. And how are the Father, Son, and Spirit one?
I have always found this illustration to be a helpful expression of the Christian doctrine of the Trinity as expressed in the historical Christian creeds and confessions. It doesn't say all there is to say on the Trinity by a long shot, but it does seem to be a true representation of the manner in which the Trinity is one. And if this is how the Trinitarian God is unified and one, then shouldn't his Church be one in the same manner? And isn't the manner of unity important, since Jesus explicitly mentions the manner in which the Church ought to be unified? And doesn't "being one as God is one" mean something different for those who believe in a Trinitarian God than for someone who believes in God as wholly singular in the strictest sense, such as the god of deism or Islam?
From the helpful illustration above I have derived the following illustration of what may be something like Christ's vision for the unity of His Church.
In addition to Christ's prayer for Unity seeming to call for a Trinitarian form of unity, other passages in the New Testament seem to call for unity in 'mind and thought' or 'love' or 'faith' (1 Corinthians 1:10, Ephesians 4:7-16), rather than unity under a single 'roof' as it were, or uniformity in every way imaginable, even on non-essential matters. Which leads me to believe it's possible that some visions of unity are misguided.
Many ecumenical efforts seem to have the long-term goal of collapsing all differentiation and diversity within the Church into univocity. And yet there is an internal dynamism, dialogue, and life within the Godhead, which is the model of unity Christ has in mind for The Church. If we don't feel the need to 'resolve' these tensions within the Godhead -- as we shouldn't -- then we shouldn't feel the need to resolve them within the Church. If our vision for church unity is utter univocity on all things, then why did Christ bother with the words that follow "as"? Shouldn't the vision of unity for a Trinitarian church differ from that of the vision were God's nature not Trinitarian? If a Muslim or a Jew had a vision for church unity which reflected the Oneness of God, it would make sense that there should be no differentiation within that church at all, but doesn't the Christian, Trinitarian God differ on this count?
Though these are the three largest branches of historic Christianity, I'm not claiming that the Church must be triune with these three specific distinct branches, or even that there couldn't be a larger number of branches. And of course under each umbrella, especially that of Protestantism, there is further differentiation still which isn't shown. The point is only to raise the question of whether 'One Unified Church in Christ' ultimately requires collapsing all of these distinctions.
Of course, this leaves open the question of which so-called divisions are not actually meaningful, sinful divisions, but are actually just an expression of the glorious life of the Trinitarian Church, and which divisions are products of sinful pride which are in need of healing. And I'm not claiming to address the question of which is which here.
The pictorial depiction of Church unity that I've given is what I imagine might be a rough approximation of how the Church can be unified, and yet still contain a manner of diversity on non-essential matters within that greater unity. In other words, that depiction doesn't show the existing divisions that are products of sin and which are legitimate barriers to Church unity. A visual depiction of those types of divisions might be a lightning bolt emanating from the space between two of the branches of Christianity, which results in "The Church" at center having a fracture in it. This type of division, of course, we can not abide and must seek to heal and overcome through the power of the Holy Spirit. My point is only that we should be mindful of the fact that our unity is to be modeled after the Trinity, and if that is the case we should attempt to make a distinction between unhealthy, sinful divisions that injure Church unity, and differentiation within the Church which reflects the life of the Trinity. Otherwise we risk wasting energy attempting to resolve some of the healthy, dynamic, lively expressions of the Trinitarian Church.
While it's noble to seek reconciliation and healing of divisions within the church in many cases, in other cases it can be problematic to imagine yourself as being 'separated' or 'divided' from brothers in Christ in the first place, when you actually aren't.