Sociologists have long been interested in how interactions among the diverse groups that constitute modern societies shape group mobilization efforts, including the use of group media. We advance research on this topic by analyzing the growth of magazines affiliated with religious groups in antebellum America, when the nation was becoming a modern society. We draw on the sociology of religion, organizations, and media to develop hypotheses linking the growth of denominational magazines to inter-denominational competition, intra-denominational fragmentation, denominationsâ€™ geographic dispersion, and denominational resource sharing across locations. We test these hypotheses using dynamic techniques on a unique dataset that includes all religious denominations and denominational magazines in the United States between 1790 and 1860. Because our analysis focuses on tools for mobilization â€“ magazines â€“ it avoids the definitional dependency between explanation and outcome that has plagued much research on religious groups. Our results show that denominations published magazines in response to both inter-denominational competition and geographic expansion. However, they used magazines in a manner more consistent with a theory of resource sharing than with ethnic-competition and religious-economies theories. And contrary to expectations, we find that intra-denominational fragmentation did not contribute to the growth of antebellum religious magazines. Our analysis not only links interactions between religious groups to broader group processes, it also offers fruitful ways to extend the analysis of other kinds of groups.