The elder “put the Muscovite prince on par with the Roman Emperor Constantine the Great, identifying the latter as the ancestor of the prince.” Such ideas became possible after the Ottoman Empire seized Constantinople, the Second Rome, in 1453. As a result, Byzantium ceased to exist as a state and “as a center of world Orthodoxy.”
Moscow thus presented itself as its successor civil and religious. In 1589, the idea of Moscow as the Third Rome became official policy and the basis for its imperial pretensions as unique and part of an apostolic succession. “Byzantium had fallen; Moscow took its place. As Filofey wrote, “Moscow is the Third Rome; a fourth there shall not be!”
The Third Rome doctrine was especially popular during the reign of Alexander III and was popularized by historian Vladimir Soloyev, who saw it as an indication that Moscow would unite East and West in itself and thus create “the so-called world unity.” As such, it led to the formation of “’the Russian idea.’”
Now, with Ukraine having achieved autocephaly, the notion of Moscow as the Third Rome has been shown to be hollow. That is not something that many Russians can easily accept; and it is why Russia will continue to fight the rights of Ukraine and why the tomos is ultimately more significant than the Beloveshchaya accords of 1991.