From The Crimean War: A History, by Orlando Figes (Metropolitan, 2011), Kindle Loc. 276-288:
More than any other power, the Russian Empire had religion at its heart. The tsarist system organized its subjects through their confessional status; it understood its boundaries and international commitments almost entirely in terms of faith.
In the founding ideology of the tsarist state, which gained new force through Russian nationalism in the nineteenth century, Moscow was the last remaining capital of Othodoxy, the ‘Third Rome’, following the fall of Constantinople, the centre of Byzantium, to the Turks in 1453. According to this ideology, it was part of Russia’s divine mission in the world to liberate the Orthodox from the Islamic empire of the Ottomans and restore Constantinople as the seat of Eastern Christianity. The Russian Empire was conceived as an Orthodox crusade. From the defeat of the Mongol khanates of Kazan and Astrakhan in the sixteenth century to the conquest of the Crimea, the Caucasus and Siberia in the eighteenth and nineteenth centuries, Russia’s imperial identity was practically defined by the conflict between Christian settlers and Tatar nomads on the Eurasian steppe. This religious boundary was always more important than any ethnic one in the definition of the Russian national consciousness: the Russian was Orthodox and the foreigner was of a different faith.
Religion was at the heart of Russia’s wars against the Turks, who by the middle of the nineteenth century had 10 million Orthodox subjects (Greeks, Bulgarians, Albanians, Moldavians, Wallachians and Serbs) in their European territories and something in the region of another 3 to 4 million Christians (Armenians, Georgians and a small number of Abkhazians) in the Caucasus and Anatolia.