 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.Cosmas the Priest's only known work, "Sermon Against the Heretics", bears the full title "Homily of the Unworthy Presbyter Cosmas Against the Newly-Appeared Bogomil Heresy" or, in other manuscripts, "Sermon of Saint Cosmas Presbyter Against the Heretics, A Discussion and an Instruction from the Books of God".It has been preserved in 25 full copies, all from East Slavic sources, and 116 excerpts or compilations, some of which are of South Slavic origin.