The historical and cultural situation of the 17th century is characterised. The role of Polish culture, the Polish and Latin languages is highlighted. They influenced the book repertoire of the translated texts of that period. The so-called scholarly Church Slavonic is characterised. It formed under the strong influence of the South Western book and language traditions. The background of book "proofreading order" introduced by tsar Alexey Mikhailovich and Patriarch Nikon is discussed. This event lead to the appearance of the Old Believers. The evolution of Church Slavonic into a more mundane language, on the one hand, and the influence of the popular spoken Russian on the written language, on the other hand, is studied along with the issue of the reorganisation of the genre system of the 17th century medieval literature. These issues are studied as aspects of the Russian literary language "democratisation" process. Protopope Avvakum's works are also analysed in this context.