This article discloses those aspects of abolishing the Brest Church Union within the borders of the Russian Empire in 1780–1875, the comprehension of which remains relevant for historical and church-historical science. The author draws the following conclusions: 1) The reunification of the Uniates with the Orthodox completed a centuries-old period in the history of Russian Orthodoxy, which can be characterized as an era of separation. Formally, the canonical division of the Russian Orthodox Church, which took shape in the mid-15 th century, was overcome at the end of the 17 th century, when in 1686, the Orthodox of Rzeczpospolita became part of the Moscow Patriarchate. However, the consequence of the canonical division — the division of the Western Russian population into Orthodox and Uniates — was overcome only in 1875, when 250,000 Uniates returned to the Orthodox faith. 2) As a result of the Uniates’ reunification with the Orthodox, most of the Byelarussian–Ukrainian population returned to the Orthodox path to their salvation and spiritual origins. This freed the Western Russians from the grafting of the alien to them Western Christian form of spirituality and Polish culture and served to develop the Orthodox forms of religiosity natural to the people in the second half of the 19 th century. Therefore, the development of distinctive Byelarussian and Ukrainian cultures as branches of all-Russian culture became possible. 3) The reunification of the Byelarussian–Ukrainian Uniates with the Orthodox was one of, if not the most, significant achievements of the Russian Empire in the lands annexed from Poland. Without this event, a lasting comprehensive integration of the Western Russian territories with the Great Russian provinces would not have been possible. 4) The termination of the Brest Pact halted the process of the involvement of the Byelarussians in the Polish Church and united the Byelarussians spiritually and culturally, having destroyed the religious and cultural boundary which divided the people, and which could be traced on the geographical map. In 1839, the Byelarussians got rid of the intrusive Romanization, Polonization and Russophobia propaganda and returned to the Russian historical and cultural world, where they got the opportunity to cultivate their identity and did not feel like second-rate people, as they did in the Commonwealth of Poland.