Bogomil teaching did not stay in Bulgaria. Missionaries and travelers carried it south and east through Byzantine Thrace and Macedonia into Constantinople itself, where by the twelfth century it had gathered enough of a following, including among the capital's own clergy and monks, to alarm the emperor Alexios I Komnenos. Alexios had the Bogomil teacher Basil the Physician arrested, interrogated at length, and burned at the stake around 1105, and commissioned the theologian Euthymius Zigabenus to compile a full refutation, the Panoplia Dogmatica, whose account of Bogomil doctrine, hostile as it is, remains the fullest single description that survives. West of the Balkans, closely related teaching surfaced among the Cathars of Languedoc and northern Italy a few decades later, most historians crediting the tie to direct missionary contact centered on the Bogomil bishop Nicetas rather than to coincidence, though the exact document recording his mission has its own authenticity dispute.
In the Balkans the movement's own trail grows harder to follow. A separate Bosnian Church emerged in the same medieval centuries, and for a hundred years historians read it as Bulgarian Bogomilism transplanted; that reading is now widely doubted, and whether Bosnia's later conversion to Islam under Ottoman rule owed anything to a Bogomil inheritance is treated with far more caution than older popular accounts allow. What is not in doubt is that organised Bogomil communities in Bulgaria itself had faded from the documentary record before the Ottoman conquest closed the medieval Balkan world in which the movement had lived for roughly four centuries.