Abstract: The article is focused on an examination of the activity of the metropolitan of Moscow and Kolomensk Macarius (Bulgakov) as a historian of Old Ritualism and a missionary. Particular attention is allocated to an analysis of the historical works of the metropolitan Macarius dedicated to Old Ritualism and his missionary activity among the schismatics and sectarians. The article studies the principal provisions of his works on the history of the Russian church that explain the causes and conditions under which appeared Old Ritualism and sectarianism, and demonstrates the main contributions of the metropolitan Macarius in the field of Old Ritualism studies. The author examines the historical-methodological concept of his studies, as well as demonstrates the significance of his works for historical science in the research of Old Ritualism and sectarianism. The article also analyzes the anti-schism, anti-sectarianism missionary activity of metropolitan Macarius as archbishop during his years of governance over the Tambov, Kharkov and Lituania dioceses. The author elaborates the statistical data and dynamic of the changes in numbers of Old Ritualists and sectarians in each of the three diocese on the basis of the statistical material from the governorates that were part of them, and presents the results of the missionary activity of the metropolitan Macarius.