Over its millennial history the Russian Orthodox Church has not had deeper and more radical changes than the Patriarch Nikon's reform in the mid-17th century. He introduced the canons for texts of the Old and New Testaments, liturgical ceremonies, clerical garment, etc, which are extant. Russian historiography characterizes Nikon as "one of the most outstanding and powerful figures in Russian history" and acknowledges the fact that the Patriarch faced enormous difficulties in the course of the reform, but also permanently considers him to be one of the main perpetrators of the church schism - a so called split in the Russian Orthodox Church into New and Old Believers. This opinion is widely held in the church milieu, but is in opposition to the facts. It is known that the essence of the reform was to unify all the Church canons (texts, liturgy, etc.) for the whole of Russia, whose territory from the Baltic to the Pacific was united shortly before the mid-17th century. Before, for centuries the dominant ethnic group had been separated and developed in dozens of different states - Duchies. Formally, the Russian Church was united, but the power of the metropolitans of Kiev, Vladimir and Moscow was purely nominal. Naturally, this led to misunderstandings liturgical ceremonies, accumulating various errors in the texts of the Bible and others, which had been copied by monks in different Duchies. Following the political unification of the country, Nikon consolidated it ideologically and spiritually. This was the essence of the reform, and the church schism was caused by the fact that some Russian parishes had not adopted the church innovations. It should be emphasized that, unlike the single Russian Church, Old Believers had not been able develop their own common canons. Old Believers not only dissented from the One Orthodox Church they were separated. It should also be noted that Nikon consolidated Russia not only for the present and future, but he also consolidated its past having created a chronicle, which is now called Nikon's chronicle. The basis for unifying texts and rituals were the canons approved by the Patriarch of Constantinople. It may be noted that firstly, the reform was supported by the authority of the places from where the Christian faith had come in Russia, and secondly, it is clearly indicated Nikon's desire to unify liturgy cannons of the Russian Church and all other Orthodox churches that prepared them to congregate under the Russian Church supervision, i.e. the reform was a significant step in the realization of the slogan "Moscow is the Third Rome". Through the example of Nikon, who was Mordvinian by birth, we can get an idea of the contribution of Finno-Ugric peoples to creating the Russian state.