The article deals with the socio-historical and axiological informativity of phraseological units in the Old Church Slavonic language in the aspect of the history of Russian phraseological system. The author pays her attention to the superword language units in Slavonic manuscripts of the 10-11th centuries. The purpose of the research is to identify through the prism of phraseology the origins of Slavonic key values, which were transformed during Christianization, and their superword nominations. The novelty of the study lies in the fact that besides the canon sources, the article analyzes the sources which were ignored by the authors of the Old Church Slavonic dictionaries. The object of the research is the Old Church Slavonic phraseological units reconstructed with the continuous sampling method from the texts created in different parts of medieval Slavia. To analyze the linguistic material, comparative-historical, linguoculturological, linguocognitive methods and methods of component and contextual analysis were used. As a result, adjustments were made to the idea of the activities of medieval Christian communities. Old Slavonic phraseological units testify that the former pagan Russians were attracted to Christians by their boundless faith in the truth of Christ’s teaching, their ability to defend their beliefs, personal courage, love for neighbor and mercy. The relevance of this study is that in the context of globalization, the Christian values assimilated by the Eastern Slavs during Christianization, which contributed to the powerful take off of Russian culture, today need to be protected. The research direction outlined in this article opens up the prospects of establishing the origins of the Russian Christian phraseological corpus and its rules.