Second Bulgarian Empire
Bogomilism and other heresies

Bogomilism, a Gnostic, dualistic sect, was founded in the 10th century during the First Bulgarian Empire. It later spread throughout the Balkans and flourished after the fall of Bulgaria under Byzantine rule. The Eastern Orthodox Church considered the Bogomils, who preached civil disobedience that was particularly alarming for the state authorities, heretics.

Bogomilism saw a major resurge in Bulgaria as a result of the military and political setbacks during the reign of Boril (r. 1207–18). The emperor took swift, decisive measures to suppress the Bogomils; on 11 February 1211 he presided over the first anti-Bogomil synod in Bulgaria, which was held in Tarnovo. During the discussions, the Bogomils were exposed; those who did not return to Orthodoxy were exiled. Despite the extant union with the Roman Catholic Church, the synod followed strictly the canons of the Orthodox Church. In the specially dedicated Book of Boril, the monarch was described as "Orthodox emperor" and the Synod of Tarnovo was added to the list of Orthodox synods. As a result of Boril's actions, the influence of the Bogomils was greatly reduced but was not eradicated.

Many heretical movements, including Adamites and Barlaamism that arrived with exiles from the Byzantine Empire, established themselves in Bulgaria in the 14th century. These movements, along with the Bogomilism and Judaism, were condemned by the Council of Tarnovo in 1360, which was attended by the imperial family, the patriarch, nobles, and clerics. There are no sources about the existence of Bogomils in Bulgaria after 1360, implying the sect had already been weakened and had few followers. Persecution of the remaining Adamites and Barlaamists continued on a smaller scale, headed by Theodosius of Tarnovo and patriarch Euthymius.
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