Presbyter Cosmas

Cosmas the Priest (Bulgarian: Презвитер Козма, Prezviter Kozma), also known as Cosmas the Presbyter or Presbyter Cosmas, was a medieval Bulgarian priest and writer. Cosmas is most famous for his anti-Bogomil treatise Sermon Against the Heretics, which, despite not being conclusively dated, is generally ascribed to the 10th century. The treatise is a valuable source on the beginnings of the Bogomil heresy in Bulgaria, as well as on medieval Bulgarian society.

The little that is known about Cosmas can be extracted from the few words that he writes about himself in Sermon Against the Heretics. As was customary to medieval priests and writers, Cosmas refers to himself as "unworthy". However, he was certainly of no low rank, as in his treatise he widely criticised the high clergy of the Bulgarian Patriarchate. Bulgarian historian Plamen Pavlov theorises Cosmas must have been a high-ranking member of the ecclesiastical hierarchy and would have written his treatise under direct orders from the Bulgarian emperor. There is no data as to where in Bulgaria Cosmas was based: suppositions range from the capital Preslav and eastern Bulgaria in general, to Ohrid and the region of Macedonia, and even Veliko Tarnovo. Though Cosmas is not known to have been canonised, he is commonly referred to as "blessed" or a "saint" in the copies of his treatise.

The dating of Cosmas' activity and thus the writing of Sermon Against the Heretics is an extremely problematic issue. The general consensus among scholars is that Cosmas lived in the middle or the second half of the 10th century. However, individual scholarly opinions associate Cosmas' life with the first half of the 11th century and even the early 13th century. While Cosmas never mentions the date of writing of his treatise, he does leave some chronological details. Cosmas calls the Bogomil heresy "newly-appeared" and refers to the apparently popular "John, the new presbyter and exarch", whom most scholars identify with early-10th-century Bulgarian writer John Exarch. Cosmas insists that the heresy spread during the reign of Tsar Peter I (r. 927–969), yet according to historian Dimitri Obolensky he also claims Peter's rule was already over by the time of writing.

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