This thesis examines the story of a thirteenth-century woman from the diocese of Metz, named Sibylla of Marsal, as the contemporary monk and chronicler Richer of Senones recounts it in his Gesta Senoniensis Ecclesiae. According to Richer, Sibylla feigned sanctity using various props--including a demon costume that she wore to terrify villagers--and was locally venerated as a holy woman before authorities discovered her fraudulence. This thesis offers the first full-length study of Sibylla and is the first study of this fascinating case to focus on Richer's perspective. After establishing the single extant thirteenth-century manuscript of the Gesta Senoniensis Ecclesiae--Paris, BnF ms. lat. 10016--as the most reliable witness to Richer's original text, this study analyzes Richer's agenda to situate Sibylla within his apocalyptic worldview and his desire to denigrate the emerging mendicant orders. Finally, Sibylla's story is placed within the broader context of thirteenth-century women's religion; because Sibylla exhibited accepted behaviors associated with female sanctity and yet was not ultimately considered a saint by her contemporaries, her story provides insight into the social construction of sainthood in the High Middle Ages. Several appendices edit and translate the crucial medieval sources for the thesis.