The Luciferian heresy appeared for the first time in the 13th century in the area round Cologne, Mainz and Trier in the Rheinland. Contemporary records suggest that its followers believed Lucifer to be a benevolent deity, unjustly banished from his heavenly kingdom. They looked forward to the moment when he would eventually reclaim his rightful position and believed that it could be hastened by their fasts and ascetic practices. So far students of medieval heresies have been sceptical about the actual existence of Luciferianism and regarded it as a fiction invented by the Inquisition to smear and utterly discredit some heterodox Christians. The view that the Luciferian heresy was no more than perfidious mud-slinging is based primarily on the records of investigations by Conrad of Marburg and his collaborators, an inquisitorial team notorious for extricating self-incriminating evidence from innocent suspects.The aim of this paper is, first, to challenge the view that Luciferianism was a mere invention of the inquisitors and, second, to find out what its characteristic traits may reveal about its origins as well as affinities and links with other heretical movements of the Middle Ages. A careful analysis of a broader range of sources - especially Pope Gregory IX's bull 'Vox in Rama', the testimony of a heretic named Lepzet, and several entries in German chronicles - reveals significant similarities between Luciferianism and Provençal Catharism on the level of doctrine, ritual and organization of the community. As in the case of Catharism, the beliefs ascribed to the Luciferians seem to represent a clear continuation of the legacy of Gnosticism. In consequence, the view that Luciferianism was but a figment of the inquisitorial imagination needs to be revised.