The book explores the process of Christianization of the Roman Empire, its basic aspects and historical significance. Christianization is studied as a historical phenomenon in its characteristic manifestations, including preconditions of Christianization, rates of dissemination of Christianity in the Roman Empire, religious policy of the Roman emperors from Constantine to Theodosius, formation of the Church as religious, ideological, social and political institution. Christianization is understood as the process of interaction and mutual influence of all the structures of the late antique civilization and the Christian religion with all its institutions. The problem of the struggle inside the Church is also examined. The survey is grounded on various sources, including the works of the Fathers of the Church, church historians, pagan authors, and the Roman legislation. The book is oriented on the positive experience, accumulated in the best works of Russian and world scholars. This book will appeal to students and scholars, as well as to all readers, interested in the History Late Antiquity and Ancient Christianity.