The monograph is the first comprehensive study in modern Russian historiography of such an important phenomenon of the spiritual and social life of the late ancient world as the Arian crisis, based on the history of one of the regions of the Roman Empire - Illyricum. In the course of theological disputes of the IV century. the most important dogma of the Blessed Trinity within the framework of the Christian tradition was fully revealed. In addition, the Arian turmoil had a significant impact on the development of church organization, canonical tradition and the relationship between imperial power and the episcopate.