This dissertation examines the ways that bodies are used in defining the boundaries between pious 'religion' and illicit 'magic' in Jewish, Christian, and Islamic literatures of the fifth to ninth centuries of the Common Era. Drawing upon narratives and legal discussions both of exceptional bodies (of martyrs, saints, rabbis, and prophets) and of average laypeople's bodies, this dissertation suggests that ritual usage of the body functions in these literatures as a site for the rhetorical construction of religious identity through the differentiation of acceptable bodily practices from those defined as unacceptably sectarian or 'magical.' By reading discussions of 'magical' bodies and bodily rituals, we see that late ancient ideas of the body's inherent power simultaneously enforced and violated the constructed boundaries between religious communities. Devoting particular attention to the usage of spittle and hair in discussions of magic and the power of the body, this project illustrates that the body was an important yet paradoxical site for the performance of religious identity and for the construction of religious difference in late antiquity. While late ancient sources draw upon the discourse of 'magic' to define as illicit those bodily performances understood as problematic and insufficiently 'orthodox,' these same bodily articulations or pieces (such as spittle and hair) might also be called upon to display ritual authority and concentrations of power in certain individuals. Spitting could signal holiness and healing, but could also be marked as an act of sectarian practice or sorcery. Hair could be a source of divine blessing, or a material for sorcerous cursing. The different valences ascribed to spittle and hair display the ambiguity of these distinctions between religion and magic in late antiquity, as well as the power placed in even these most effluvial bodily parts. Late ancient sources map a variety of discursive categories onto these bodily pieces and the distinctions between religion and magic, or orthodoxy and heresy, often hinge on variant usages of these corporeal items. The efforts to define the proper usages of the body-including even spittle and hair-highlight the late ancient image of the body as standing on the edge of religion and magic, holiness and heresy, health and illness, power and weakness.