The founding editors of Christianity Today spent more than a year planning the launch of their magazine. Carl F. H. Henry, L. Nelson Bell, and J. Marcellus Kik believed Christianity Today could “plant the flag” for evangelicalism. To do that, though, the editors had to decide what evangelicalism was. They had to decide where the lines were, who was in and who was out, which issues mattered and which did not. One key criterion, they decided, was whether or not someone liked evangelist Billy Graham. Historian George Marsden later offered this as a tongue-in-cheek definition of evangelicalism. More seriously, religious historians have used David Bebbington's quadrilateral definition, which says the basis of evangelicalism is conversionism, biblicism, activism, and crucicentrism. This article argues that Bebbington's definition is ahistorical, vague, and deeply unhelpful. Marsden's joking definition, on the other hand, is quite useful, as it directs historians to attend to actual relationships, historical affinities, and real-world conversations. Based on new archival research, this article tells the story of the launch of evangelicalism's “flagship” magazine, shows how evangelicalism's lines were drawn in 1956, and makes the case that evangelicalism is best understood as a discourse community which is structured by its communication networks.