Among the most intricate historical and religious mysteries of medieval Europe are those posed by the "Great Heresy" - the sudden rise and spread of medieval dualism, the belief that cosmos and man are constant battlegrounds between the forces of good and evil and their supernatural protagonists. This book offers a comprehensive history of religious dualism, from its early expressions in late Egyptian religion and the revelations of Zoroaster through the Dead Sea Scrolls, the Mithraic Mysteries, and the Great Gnostic teachers to its revival in medieval Europe and the suppression of the Bogomils and Cathars who were seen as heirs to that ancient rival of Christianity, Manichaeism. Exploring crucial stages in the history of Christian dualist heresy, Yuri Stoyanov illuminates a variety of religious and political undercurrents that lie beneath the surface of recorded history.