The idea that American Protestantism has a mainline, or ever did, was born in irony. The mainline is usually identified with seven Protestant denominations, all of which were small in their heyday of the 1950s. Moreover, just after they acquired the name mainline, they began to shrink. The idea of the mainline had a basis, but it had little to do with numbers. The mainline churches assumed a leadership role in American society, they built a large stock of cultural capital, they crafted a persuasive rhetoric about modern Christianity and America, they built an ecumenical national church, they were (and still are) overrepresented in the corridors of power, and they served as guardians of Americas moral culture. One of the best ways to see how they did it is to track the history of the Christian Century magazine, the flagship of mainline Protestantism.
For many years I have been telling colleagues and graduate students that somebody ought to write this story. Nobody understood twentieth-century ecumenical Protestantism better than the founder and editor of the Christian Century, Charles Clayton Morrison. Though forgotten today, he influenced this tradition more than almost anybody. In addition, Morrison wrote an unpublished autobiography chock-full of insights about his ambitions for the Century and how he kept it going.