The Byzantine Empire, with its capital at Constantinople ("Second Rome"), saw itself as the natural successor to the Roman Empire. When Constantinople fell to the Turks in 1453, the Russian ruler Ivan III began to popularise the notion that Russia was the successor to the Byzantine Empire, and therefore the "Third Rome". Russia borrowed much of its culture, including many of its Orthodox religious beliefs and traditions, from the Byzantines. Since Russia was the last major independent Orthodox nation after tossing out its Tatar overlords in 1480, the sentiment would be promoted for a long time.
It was-around the 1500s-because Russia was last remainder of the Christian civilization, most of which had succumbed to heresy. Both Roman Catholicism and Islam were considered heretical offshoots of the Christian stem by many Orthodox believers.