All Christians are members of one another because of their union with Jesus Christ, their common Head. There is therefore but one Church: holy, catholic, and apostolic. Accordingly, striving to bring into a fuller outward expression this unity which we already share in Christ is the duty of every Christian, commensurate with the command of the Apostle (Ephesians 4:3).Notice the phrase "fuller outward expression". According to Jonathan's position, the ground of the unity of the Church is inward and spiritual; it is each believer's internal/spiritual union with the invisible Christ. That internal/spiritual unity is already fully there, and so ecumenicism is merely an effort to further manifest outwardly what is already fully there inwardly.
I want to contrast Jonathan's position with the Catholic position. (Let me say in advance that I respect Jonathan, and agree with him on many points, and I have had some profitable and friendly discussions with him. So my criticism of his position on Church unity is no way meant to be a personal criticism of him.) The Catholic position on Church unity is not that the Church's unity is fundamentally internal and spiritual. For the Catholic, the ground of the Church's unity is Christ, who is both spirit and flesh. We are united to Christ by being united to His Body (i.e. the visible Church) through the sacrament of baptism ("water and the Spirit"). We are more deeply united to Christ and the Church through the sacraments of Confirmation and the Eucharist. From the Catholic point of view, those communities lacking valid orders lack these two sacraments, and therefore are less united to the Church (the Body of Christ), and hence less united to Christ. Moreover, in the Catholic view an act of schism separates a person (to some degree) from the Church, and hence from Christ. Jonathan's view, by contrast, treats all Christians as equally united to Christ and therefore equally united to the Church. For that reason, on Jonathan's view, schism does not do anything to the internal unity of all Christians, only to the outward manifestation of that internal unity. But the Catholic view looks at it the other way around, in the earth-to-heaven direction ("whatever you bind on earth will be bound in heaven"). From the Catholic point of view, to separate to some degree from the Church by way of schism is to separate to that degree from Christ. The visible and the invisible are bound together, because of the incarnation, wherein what is done to the flesh of Christ is done to the Person of Christ. That is precisely why excommunication has teeth; it means something. It really cuts us off from Christ. If it did not cut us off from Christ, then it would have no teeth.
Jonathan's view of the ground of the unity of the Church is de-materialized (i.e. spiritualized). It is in that way both gnostic, as I described here, and Docetic. In actuality, Christ is both spirit and flesh. Visible unity is not merely an "outward expression" of the real unity, just as sexual union is not merely a physical expression of inward/spiritual unity. Sexual union is part of the true union of husband and wife. Likewise, visible unity is part of the real unity of the Church, not merely an outward expression of the real unity which is spiritual and invisible. From the Catholic point of view, ecumenicism is not in effort to bring an outward expressions of a unity that is already complete in the invisible / spiritual realm. Rather, ecumenicism is the work of bringing those only in partial communion with the Church (and thus only in partial communion with Christ) into full communion with the Church (and thus into full communion with Christ). Jonathan's view essentially denies the actuality (and possibility) of real divisions in the Body of Christ; his position essentially denies that any believers are not fully united to the Church. According to his position all the various denominations are not *really* divided; we are divided only in outward expressions.
It is no wonder there is so little concern about Church unity in these communities, if our disunity is viewed in this gnostic manner as merely a matter of outward expressions. The root problem here is an implicit dualism that treats the spiritual as the really real, and the material as a mere context for the expression of the spiritual. Likewise, and for the same reason, Jonathan's position treats the Body of Christ as fundamentally invisible, but having some visible members, whose activities are often visible, and thus are "outward expressions" of the fundamentally invisible Church. Wherever being in schism is treated as not separating a person (to some degree) from Christ, there the Church is being treated as fundamentally invisible. (If the unity of the Church is fundamentally invisible, then the being of the Church is fundamentally invisible, because of the metaphysical relation of being and unity.) This dualism (or 'angelism' as Maritain calls it) in Jonathan's position can be traced back to Descartes. In actuality, we are neither angels nor heaps of atoms. We are rational animals, both spirit and flesh. To be fully united as one Church, it is not enough for us all to believe in Jesus, just as the gnostics were wrong that salvation is through knowledge (something intentional, immaterial and invisible). We must be one family, one community, one visible body, and that requires that we be under one visible head. Hence the incarnation was essential for Church unity. In order for the Church to remain truly unified when Christ ascended into heaven, Christ had to give primacy to one of the Apostles, and to his episcopal successors. And this is why Christ made Peter the steward of His Church until His return.
And the Lord said, "Who then is the faithful and sensible steward, whom his master will put in charge of his servants, to give them their rations at the proper time?" (Luke 12:42)
Thus says the Lord GOD of hosts, "Come, go to this steward .... I will entrust him with your authority, And he will become a father to the inhabitants of Jerusalem and to the house of Judah. Then I will set the key of the house of David on his shoulder, When he opens no one will shut, When he shuts no one will open. (Isaiah 22:15,21-22)
I also say to you that you are Peter, and upon this rock I will build My church; and the gates of Hades will not overpower it. I will give you the keys of the kingdom of heaven; and whatever you bind on earth shall have been bound in heaven, and whatever you loose on earth shall have been loosed in heaven." (Matthew 16:18-19)