What are the reasons for such a rapid spread of Christianity?

a) This religion best corresponded to the feudal system and was actually taken for the state in the Roman Empire, especially Byzantium. The first circumstance contributed to the fact that Christianity was adopted by Kievan Rus. b) The Church, having received, including secular power on the territory of the former Western Roman Empire, began actively (and not unsuccessfully) to spread its faith, filled the spiritual vacuum, which then there was simply no one to fill. C) The assimilation of the New World, the active participation of Christians in this, also played a significant role.