The Third Rome
In 1453, Muscovite tsar was the only legitimate Orthodox ruler when Constantinople had fallen to the Ottoman Empire.
Moscow was the Third Rome because it was the center of Christianity succeeding Rome and Constantinople. This concept spread to later Russians too.
The Title of "Tsar" & "Sovereign of All Rus"
Tsar which came from 'Caesar' was the title of Ivan III. Since Moscow claimed the Third Rome to the legacy of the Byzantine Empire, he emphasized it with using the title 'Tsar.'
Also, Ivan III was the first person to use the title "Sovereign of All Rus."
(His success in making Moscow the centre of Russian power)
<Great Zion> is made in 1846. It raises representations of 12 apostles. The interior contains an inscription that <Zion> was made at the decree of Ivan III. Ivan III was not only Grand Duke of Muscovy, but also "Sovereign of All Rus".