The Christian Century is generally regarded as the most influential Protestant magazine of the early twentieth century. But what does it mean to call the Century influential? Whom did it influence, and how? This article takes a historical approach, examining the size of the Century's audience, the nature of the magazine's impact on its readers, and the ability of those readers to extend the magazine's impact beyond themselves. The main source for this investigation is a cache of more than 2,100 unpublished letters collected in 1928 to celebrate Charles Clayton Morrison's twentieth anniversary as editor. In subscribers' own words, the letters illustrated the Century's role as a crucial link among readers with similar backgrounds and aspirations, as well as the limits of its reach beyond this cohort. The article argues that the Century was influential, but not because it converted (directly or through its clergy readers) large numbers of American Protestants to its progressive vision for Christianity. Rather, the Century was primarily influential in the process of mainline identification, both in the sense of defining which writers, institutions, and ideas belonged to the emerging mainline tradition and in the sense of offering readers an opportunity to identify with that tradition. A better understanding of the Century's influence during the Morrison era sheds light on the rise of the mainline. Additionally, a better understanding of the kinds of influence magazines do and do not exercise is helpful for anyone who looks to periodicals to provide a barometer of cultural trends.