Complex sentence: The Christianization of the Rus' Khaganate is supposed to have happened in the 860s and was the first stage in the process of Christianization of the East Slavs which continued well into the eleventh century.
Split into simpler sentences: The Christianization of the Rus' Khaganate is supposed to have happened in the 860s. It was the first stage in the process of Christianization of the East Slavs which continued well into the eleventh century.