This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
Christianity becomes the state religion of the Rus through the baptism of Vladimir, grand prince of Kievan Rus, traditionally dated to 988 and carried out, by most accounts, at Chersonesos in Crimea. His own baptism was followed directly by the mass baptism of Kiev and the adoption of Byzantine rite Christianity across his realm, ending the state sponsored pagan pantheon that Vladimir himself had earlier promoted before his conversion.
The conversion was bound up tightly with politics rather than religion alone. Vladimir supplied military support to the Byzantine emperor Basil II and married into the imperial family as part of the same arrangement, which is exactly why the religious and political chronology of these events sit so tangled together in the surviving sources, each element reinforcing rather than following the other in any clean sequence.
Both modern Russia and modern Ukraine trace a shared national and religious origin to this single event, and both claims are named here rather than resolved in favor of either. What this changed for Christianity's spread was the conversion, at a stroke, of one of medieval Europe's largest states, replacing a state sponsored pagan cult with Byzantine rite Christianity as the religion of the Rus and setting the course for Eastern Orthodoxy's later history across the whole region.