Answer
Vladimir the Great, the grand prince of Kievan Rus', made Christianity the state religion of Kievan Rus' in 988 AD. Before this, the people of Kievan Rus' practiced a polytheistic religion, with many different gods and goddesses. Vladimir was baptized in Kherson, Crimea, and after his baptism he returned to Kiev and ordered all the people of Kiev to be baptized as well.