The chroniclers of medieval Rus were monks, who celebrated the divine services of the Byzantine church throughout every day. This study is the first to analyse how these rituals shaped their writing of the Rus Primary Chronicle, the first written history of the East Slavs. During the eleventh century, chroniclers in Kiev learned about the conversion of the Roman Empire by celebrating a series of distinctively Byzantine liturgical feasts. When the services concluded, and the clerics sought to compose a native history for their own people, they instinctively drew on the sacred stories that they sang at church. The result was a myth of Christian origins for Rus - a myth promulgated even today by the Russian government - which reproduced the Christian origins myth of the Byzantine Empire. The book uncovers this ritual subtext and reconstructs the intricate web of liturgical narratives that underlie this foundational text of pre-modern Slavic civilization.