Vladimir of Kiev came to power as a vigorous pagan, and the Primary Chronicle records him setting up idols on the hill above the city before he set them aside. In 988 he was baptised and required the population of Kiev to be baptised in the Dnieper, and the eastern Slavs entered the Byzantine church.
The politics are visible in the sources. Vladimir supplied troops to the emperor Basil II at a moment of crisis, and received in return the emperor's sister Anna in marriage, an extraordinary concession for a Byzantine princess and one made conditional on his baptism. A ruler acquiring a prestigious religion, an imperial marriage and an alliance in the same transaction is not an unusual pattern in the conversion of tenth-century Europe.
The choice of Constantinople over Rome carried consequences that outlasted the politics entirely. It brought the Slavonic liturgy created by the mission of Cyril and Methodius, and with it an alphabet, a literary language and a body of translated Greek writing. The religious, artistic and legal culture of Russia, Ukraine and Belarus follows from this decision, as does their alignment with the Eastern rather than the Western half of Christendom when the two divided.