One of the greatest segments of the polyhistorian Ivo Pilar’s intellectual heritage is his attempt to interpret the phenomenon of the Bosnian Church (‘Bogomiles’). In his striving to shed light on the emergence of this religious sect, he analysed the phenomenon within a historical-religious context and as a social and political problem. According to Pilar, the religious movement in question originated from the Iranian religious reformer Zoroaster. Regardless, this dualistic religious teaching did not arrive to the Balkans in its original form but as a Manicheanism. Relying on the claims of early historiography (F. RaËki) that Bogomilism arrived to Bosnia from Bulgaria via Serbia, Pilar maintained that it also introduced “national language” and an “autonomous church organisation” as its positive values. The negative aspect of Bogomilism, which issued from the very essence of its dualistic teachings (i.e. hatred of all that which is material as the doing of Satan), undermined the very foundations of the medieval Bosnian state. For, the Bosnian state emerged, survived and fell together with its heretic Church, since, allegedly, its religious followers accepted Islam, the religion of their Turkish conquerors, in masses. Understanding Bogomilism as a purely “Croatian movement” Pilar concluded that this dualistic sect created a separate Bosnian state (but not a separate “Bosnian nation”) and, accordingly, “Bosnian separatism”, which prevented the existence of a unified Croatian geopolitical region and the constitution of a Croatian sovereignty. The origins of such perceptions of the negative consequences of Bogomilism in Bosnia lie in Pilar’s geopolitical ideas on the status of the Croatian states within the European political constellation of his time.