The Byzantine was the Eastern Roman Empire on Constantinople until the 15th century, and practiced Orthodox Christianity.
Kievan Rus was a state of East Slavic & Finnic peoples in Europe, from Belarus, Ukraine, and Russia; Vladimir the Great, (prince of Kiev), converted to Christianity to marry the Byzantine emperor's sister and formed an alliance with them; this pushed the Christianisation of of Russia embracing Eastern Orthodoxy.
The term Tsar, also spelled czar, or tzar was the Russian form of the Roman imperial title "Caesar", used to appoint supreme rulers of Eastern Europe, particularly the Byzantine ones, as the heads of the Orthodox Christian world, meaning emperor or king in Russia until 1917.