Saints have always been central to Christianity, offering models of how to follow Christ, a parallel that became resonant (and urgent) whenever this involved self-sacrifice, especially the shedding of blood. This introduction, however, suggests that ‘thinking with saints’ became an especial obsession in nineteenth-century Britain, arguing that not just Catholics but Protestants, agnostics and unbelievers, too, invested enormous time and energy into ‘thinking with saints’: using their lives and their legends to debate crucial questions about religious authority, history, miracles, and testimony. Three contexts are offered against which to read the essays that follow. The first section examines the ambivalent legacies of the Reformation for nineteenth-century ideas about sainthood, demonstrating the continuing importance of ideas about the holiness of temporality, materiality and place that Protestants repudiated officially but never entirely eschewed. The next identifies the 1840s as a key moment in which deepening confessional divisions sparked intense debate about how to use the saints of the past, and what saints might look like in the present. The final section explores the enduring importance of sainthood for agnostics and unbelievers. It examines how secularists remoulded sanctity to fit new notions of morality. And it also shows that as the twentieth century dawned, the ‘holy’ and the numinous remained live categories that science promised to develop rather than to debunk.