Poslanie mnogoslovnoe, an example of Muscovite antiheretical polemics, can be analyzed to add to our substantive knowledge of pre-Petrine Russian history and perhaps to refine our methodological tools for a critical evaluation of the sources for this period. Poslanie has long been regarded as a principal source of information about the life and ideas of its presumed author, the midsixteenth century Novgorodian monk Zinovii Otenskii, whose accomplishments as a theologian and publicist of the medieval Russian Orthodox church have traditionally been ranked only behind those of Iosif Volotskii and Maksim Grek. In conjunction with Zinovii's Istiny pokazanie, Poslanie provides our only detailed exposition of the teachings of the runaway slave Feodosii Kosoi, whose alleged criticisms of state and church constitute the most extreme rejection of the established order articulated in medieval Muscovy. In addition to its value for the interpretive study of Russian religious and intellectual history, Poslanie presents us with a methodological problem typical of early Russian documents: The work is anonymous and undated, and basic questions about its authorship, time of composition, and provenance have not been satisfactorily answered. The lack of information on either Zinovii or Kosoi and the contradictions in the pictures of both Zinovii and his heretical opponents presented in Poslanie, Istiny, and other, less detailed works, attributed to the monk, make solving these problems all the more interesting