Barton Stone, a founder of the tradition I call home, spoke of Christian Unity as our "polar star." Thomas Campbell wrote that division among Christians was a "horrid evil." To Campbell the "Church of Christ upon earth is essentially, intentionally, and constitutionally one; consisting of all those in every place that profess their faith in Christ and obedience to him in all things according to the Scriptures, and that manifest the same by their tempers and conduct, and of none else; as none else can be truly and properly called Christians." Despite this heritage, even this tradition has been fraught with division.
The church has always existed between a concern for unity and the reality of division. A gentler way of putting it would be to say that pluralism has been our common experience. But, unfortunately, this pluralistic experience hasn't always been gentle. Christians have been known to turn on each other with exceeding violence. At the same time, there have been, on a regular basis, voices of reason calling for unity. One of those voices was Eugene Carson Blake, Stated Clerk of the United Presbyterian Church in the United States [predecessor to the Presbyterian Church (USA)], who was invited to deliver a sermon in Grace Cathedral in 1960. In that famous sermon Blake called Presbyterians (at least the Northern Branch), Methodists, Episcopalians, and the United Church of Christ, to come together as one church. From that sermon a movement/organization came into existence -- the Council on Church Union (COCU). Later on other church bodies would join with those named by Blake, including the Disciples and the three African American Methodist bodies.
Keith Watkins, who is writing the history of COCU, has made excerpts of this sermon available at his blog. I would like to invite you to check out this offering, to see what was being espoused. It is interesting to note that Blake sought to bring together Reformed and Catholic impulses together. He recognized the value of a common liturgy, but knew that such a thing could not be imposed. He also recognized that if a united church was to emerge it would have to allow for theological diversity. What he thought could be the foundation would be common recognition of ministry, suggesting that the churches embrace the principle of apostolic succession.
What is interesting is that fifty years later we still haven't figured out how to handle apostolic succession. My tradition, which now has a General Minister and Regional Ministers, who function in ways as bishops, struggles with the idea that these leaders have an episcopal role, and would likely find the principle of apostolic succession foreign.
COCU finally gave way to another movement, Churches Uniting in Christ (CUiC) about a decade ago. It never fulfilled its promise, and yet it did raise the standard of unity. Perhaps in our post-modern era, such a thing as a united church isn't necessary. But, maybe we would benefit from heeding the call of those who came before us and raised the standard of unity.
Keith has yet to find a copy that can be produced in full, but the excerpts are a useful introduction. (http://keithwatkinshistorian.wordpress.com/2010/12/06/a-sermon-to-transform-the-american-church/)