The Christian Century was widely regarded as the most influential Protestant periodical in America during the first half of the twentieth century, and that perception continues among religious historians. But repeated assertions of influence do not explain how the Century rose to prominence, how it exercised its influence, or whom it affected. I intend to explore these questions, drawing on published issues of the Century, the unpublished autobiography of longtime editor Charles Clayton Morrison, and a large collection of unpublished letters from subscribers, as well as secondary sources. A history of the Century will reveal much about American religious journalism and about the Protestant mainline. Regarding journalism, my hypothesis is that the Century, as a non-denominational, subscription-supported magazine, helped create a constituency that came to be recognized by insiders and outsiders as “mainline.” Creating a constituency is one of the things magazines do best. Regarding the mainline, my hypothesis is that between 1908 and 1947 the Protestant mainline coalesced into a somewhat insular movement defined against other strains in American religion. The Century did not drive this whole process, but it can be read as a barometer of how, when, and perhaps why the “mainline” diverged from the “mainstream.”