The work of the famous Russian historian of the Christian church has long become a classic, although it remained little known to a wide circle of readers. She is distinguished by a deep insight into the historical development of the church with the complex and contradictory dynamics of the formation of dogma, organizational structure, canonical rules, liturgy and sacramental practice. The author, using historical, linguistic and theological material, reveals the complexity and inexhaustibility of the patristic heritage of the first ten centuries (before the schism of 1054) of church history, when the foundations of ecclesiasticality were laid, which determines the life of Christianity in our days. Professor Mikhail Emmanuilovich Posnov (1874–1931) He graduated from the Kyiv Theological Academy and subsequently maintained constant contacts with Western universities. He was a professor in Kyiv, and later in Sofia, where he lectured on dogmatics and, especially, church history. The book offered here is a generalized work, which he himself intended to revise and publish again. The death that befell him in Sofia in 1931 prevented him from completing the final finishing of this work, which appeared in an abridged edition in Sofia in 1937.