This essay explores the human stakes of theorizing religion in the early nineteenth century, on the borderlands of an expanding U.S. empire. It does so through the lens of a single text, A Narrative of the Captivity and Adventures of John Tanner (U.S. Interpreter at the Saut de Ste. Marie) during Thirty Years Residence among the Indians. Published in 1830, the Narrative offers an entrée into the circulation of knowledge and debates about religion among Native Americans and white settlers in a time and place from which we have little record of such debates. Tanner joined in the Midewiwin of the Ojibwe and cultivated the Anishinaabe practice of medicine hunting; held back his own skepticism, perhaps retrospectively exaggerated, at the messages of those he called “Indian prophets”; and discussed the differences, solidified in the telling, between white and Indigenous religions. His editor, Edwin James, meanwhile, drew on comparative scholarship about mythology and religion around the world to defend his own preferred theories about the religious and racial character of Indigenous peoples. Religion has long been theorized far beyond the academy and the centers of empire. Relatively unfamiliar accounts, like Tanner's, reveal how everyday people have engaged with these theories and the consequences of these theories on the ground. Tanner's Narrative, in short, usefully illuminates the webs of knowledge about religion in early America and its human stakes for people caught in the crosshairs of a transforming imperial world.