Christianization of Kievan Rus'
https://en.wikipedia.org/wiki/Christianization_of_Kievan_Rus%27
The Christianization of Kievan Rus' took place in several stages. In early 867, Patriarch Photius of Constantinople announced to other Orthodox patriarchs that the Rus', baptised by his bishop, took to Christianity with particular enthusiasm. Photius's attempts at Christianizing the country seem to have entailed no lasting consequences, since the Primary Chronicle and other Slavonic sources describe the tenth-century Rus' as firmly entrenched in paganism. Following the Primary Chronicle, the definitive Christianization of Kievan Rus' dates from the year 988 (the year is disputed[1]), when Vladimir the Great was baptized in Chersonesus and proceeded to baptize his family and people in Kiev. The latter events are traditionally referred to as baptism of Rus' (Russian: Крещение Руси, Ukrainian: Хрещення Русі) in Russian and Ukrainian literature…