Ecumenism is tough sledding. But then, I have lived long enough to see Germany re-united, the fall of the Soviet Union, and Republican control of both houses of Congress and the Presidency; surely, the millennium cannot be far behind (or something like it). All joking aside, momentous things have a way of catching people by surprise. They appear impossible right up until the moment they happen; afterwards, they appear inevitable.
So, for what it's worth, here is my vision of what the reunion of Christendom would look like.
I see a Grand Reunion Council, attended by most of the major traditions of the faith. And I see an outline of a settlement, the three major parts of which are as follows.
1. The Filioque.
EO objections to this run deep; nevertheless, Eastern and Western theologians are close to agreeing on a formula that would bridge the differences. The procession of the Holy Spirit from the Father "and the Son" was uncanonically inserted in the Creed by Visigoths converted from Arianism, as a rejection of their former error.
This was intended to safeguard the full deity of the Son. The Eastern Churches objected that it was a) done without full consideration by a Council of the whole Church, and b) implied that the Son shared with the Father the property of being the source of the Life which is in God (that is, that the Father alone is the source of "godness" communicated to the Son and the Spirit, who though co-equal and co-eternal nevertheless derive their being from the Father).
I see the Grand Reunion Council officially inserting the Filioque into the Creed, with the understanding that the Spirit proceeds from the Father "through the Son." The Spirit, like the Son, derives his being from the life which is in the Father; however, since without the Son, the Spirit cannot express himself, then from all eternity the Spirit has proceeded from the Father AND (in the sense of THROUGH) the Son.
2. The Pope.
The Roman Catholic Church has backed itself into a terrible corner with its doctrines of papal infallibility, papal supremacy, and the Marian dogmas declared by the Pope. To compromise even a little means to give up the whole game. Yet there is no getting around this.
I see the Grand Reunion Council recognizing the unique leadership position of the Bishop of Rome. He would be given presidency (though not universal jurisdiction) over the Church. In return, he would give up claims and doctrines that have made of the Pope the great stumbling block to reunion that he is (I'm quoting Paul VI on that, not bashing the papacy).
3. The Protestants.
It's hard to talk about "Protestants" in general. The first generation of Protestants took several different paths to solving what their source of authority would be upon separation from the Roman Catholic Church. Subsequent movements have multiplied those paths. New denominations have been born that have no sense of having separated from anybody: they have imagined themselves to be at liberty to begin the Church anew. But regardless of how many or how few of the major Protestant traditions would like to be part of a reunited Church (and would pay the price in doctrine and discipline to do so), I think generous provision must be made for the return of all the children of the Church.
I think each of the major Protestant traditions should be considered as an autocephalous Church or Order within the whole Church. At first, perhaps only the Anglicans, the Methodists, and the Lutherans would be eager to join the reunited Church; however, I think we would need to create a process which would allow other major bodies to join the whole, with the same status.
Perhaps some bodies would never join in. Even under the best possible circumstances, the Church may never be completely reunited. But any step forward is a step toward fulfilling Christ's prayer that we may all be one, even as he and the Father are one.
Such a Grand Reunion Council and such a settlement would require much from everybody seeking the unity of the Church. Orthodox, Catholic, and Protestant alike would all have to die to their denominational Selves in order to be reborn and raised together. But unless we are willing to embrace that death, there will be no new life together before Christ returns. After that, the problem will be solved, indeed, and we shall all have to give an account of the good that we should have done, but left undone.