This dissertation is a case study in medieval religious life. Using a remarkable collection of extant sources, including inquisitorial records, episcopal protocols, and castellan's rolls, I examine an exciting period in the religious history of Chieri, a town located near Turin in the Italian Piedmont, between the mid-1360s and 1412. These decades saw the rise and fall of a non-Catholic movement whose participants rejected almost every doctrine of the Catholic Church and believed that the devil ruled the world. More than one hundred people participated in the movement, and it received the attention of at least three inquisitors. Seventeen people (fifteen of them already dead) were condemned and burned. During that time period, there were also some key developments within the local Catholic Church. In 1383, twelve laymen founded the town's largest hospital, named in honor of the Virgin Mary, and the speedy participation of many donors ensured that the hospital became almost a public work of piety. In 1399, a large number of Chieri's people turned out for an extraordinary procession, crying "Mercy" as they marched to the all the town's churches. Finally, in 1405, under the leadership of the cathedral chapter, the community began a major reconstruction of the Duomo, the town's most important church. I argue that these key developments in the local Catholic Church were intimately linked to the story of Chieri's non-Catholic movement. At first, Catholic and non-Catholic religiosity flourished side by side, yet later, when the community renewed its commitment to Catholicism at the turn of the fourteenth century, the non-Catholic movement disappeared. Further, I argue that individuals used Chieri's religious environment to craft distinctive, personal religious experiences.