The article provides a comprehensive analysis of the nature of the “Moscow – Third Rome” theory. Particular attention is paid to the events that became key in the development of this theory, the statesmen who contributed to its formation, flourishing and oblivion, as well as the influence it had on domestic political and legal thought, which is manifested in the specific actions of the central characters of the historical scene. The author comes to the following conclusions: the theory of “Moscow - the Third Rome” had a strong influence on the political and legal life of the Russian state during the period from the reign of Ivan the Terrible to the reign of Peter I: a decision was made to crown the Grand Duke of Moscow as king, for which a special rite was developed crowning of the kingdom according to the Byzantine model, Moscow began to position itself as a protector of the Orthodox population, which led, in particular, to the capture of Kazan in 1552, the Byzantine system of “symphony of powers” developed in Rus’, the state was actually ruled by the tsar as a representative of secular power and the metropolitan as a representative of spiritual power, which was embodied in practice in the convenings of the Zemsky and Stoglavy Councils, which had the same legal force, the patriarchate was established in Rus’, while the enthronement of the first patriarch of Rus’ was carried out by the Patriarch of Constantinople, to unify the church rites of Rus’ and Byzantium, a reform of Patriarch Nikon was carried out, leading to a split and the emergence of such a movement as the Old Believers.