It is well known that the rise of anthropology in the nineteenth century existed in a tense relationship with theological approaches to the bible and Christian origins. But it remains uncertain when, why, and with what consequences European intellectuals began to apply techniques derived from an anthropologically-led 'science of religion' to interpreting the history of Christianity after the close of the patristic era. This paper focuses on the secular Jewish French anthropologist, Salomon Reinach, and his bestselling 1909 work, Orpheus, in order to examine a spectrum of critical positions concerning the extent to which anthropology could explain Christian history. It relatedly considers the ways in which anthropology informed a debate about the moral and social authority of Christianity in early-twentieth-century Europe. Locating Reinach's work in relation to contemporary movements in British and German scholarship, the paper uses a historical approach to investigate how secularist and religiously radical anthropologists used religious science to define the present-day authority of religion in relation to what they understood to be long-term processes of socio-religious transformation. It draws attention to the complexity of the encounter between freethought and religion at a period of widespread debate about the cultural and political influence of Christian ethics and the churches.