The scholarly study of new religious movements focuses on the contemporary period, but religious innovation is nothing new. This Element explores a historical epoch characterized by a multitude of emergent religious concepts and practices - the Hellenistic and Roman periods. A precondition for the intense degree of religious innovation during this time was a high level of cultural exchange. Religious elements crossed porous cultural borders and were adapted to suit new purposes. The resulting amalgams were presented in a vast corpus of texts, largely produced by a literate elite. Charismatic leaders played a particularly important role in creating new religious options and were described in genres that were infused with ideological agendas. Novel religious developments were accepted by the Roman authorities unless suspected of undermining the social order. The rise of one of the many new religions of the period, Christianity, ultimately changed the religious landscape in profound ways.