This paper analyzes the difference between bookish and colloquial church lexis in Old Russian writings. In East Slavic texts, church terms borrowed from South Slavs are to variable degrees bookish and tend to be used in different genres. East Slavic chronicles, colophons of scribes, edificatory letters of hierarchs, questions to hierarchs and their canonical responses, collections of canonical rules and penitentials, as well as birch-bark letters share a stable configuration of colloquial church terms: čьrnьcь ‘monk’ — popъ ‘priest’ — postъ / gověnie ‘fast’. Old Russian written usage shares this feature with the colloquial register. In Church Slavonic monuments created in Kievan Rus’ other configurations are represented. The paper shows that East Slavic colloquial terms — mostly of Moravian provenance — were widespread in East Bulgarian writings of the 10th century, while they are rare or absent in Church Slavonic texts translated in Rus’ during the 11th century (The Studite Typicon, The Canonical Responses of Metropolitan Ioann II). Against this background the suggestion is made that these terms were adopted from Eastern Bulgarian speakers alongside the liturgy and church practice.