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