Christian dualism originated in the reign of Constans II (641-68). It was a popular religion, which shared with orthodoxy an acceptance of scriptual authority and apostolic tradition and held a sacramental doctrine of salvation, but understood all these in a radically different way to the Orthodox Church. One of the differences was the strong part demonology played in the belief system. This text traces, through original sources, the origins of dualist Christianity throughout the Byzantine Empire, focusing on the Paulician movement in Armenia and Bogomilism in Bulgaria. It presents not only the theological texts, but puts the movements into their social and political context.