Original: Founded and ruled by a Varangian warrior elite and their descendants , the predecessor of modern Russia , Kievan Rus ' , arose in the 9th century and adopted Orthodox Christianity from the Byzantine Empire in 988 , beginning the synthesis of Byzantine and Slavic cultures that defined Russian culture for the next millennium .
Simplified: The Vikings and their descendants founded the first East Slavic state of Kievan Rus ' in the 9th century . They adopted Christianity from the Byzantine Empire in 988 .