In recent years, the growth of ethno-confessional conflicts has been observed all over the world, in connection with which the question of the role of religion in integration processes is being increasingly raised in modern Europe. In particular, the scientific interest in the history of European integration has increased, starting with the period that is identified as the period of the formation of the “European idea”, up to the present, the epoch of the emergence and development of the European Union. The purpose of this article is to study the earliest stage of development of European integration ideas, from the IV to the XIV centuries, from the beginning of the adoption of Christianity as the state religion of the Roman Empire until the crisis of medieval Christianity and the emergence of humanistic projects and the birth of the first secular projects of the union of European states. This period is characterized by the supremacy of the Christian church in the spiritual and political life, the union and unification of the management system of European nations, the formation of a common European Christian culture and worldview. At the same time there was a confrontation between the two centers of Christianity, on the one hand, and between the spiritual and secular authorities, on the other. The movement from the Christian Ecumenical Theocracy, with its center in Europe to a union of secular European states, is also an important feature of the period. The study is based on the principles of historicism and scientific objectivity, historical genetic, comparative-historical scientific methods. The article presents a study based on an interdisciplinary approach.