Schaff, Philip. History of the Christian Church (The Complete Eight Volumes in One). Amazon Kindle Edition, 2014.
Volume 2, Ante-Nicene Christianity A.D. 100-325, “Chapter 4. Organization and Discipline of the Church” Loc. 13792-14769 (part 5).
§49. Beginnings of the Metropolitan and Patriarchal Systems.
Schaff comments that a shift of Church leadership into distinct ranks was a gradual process. It was not arranged by decree (Schaff 2014, Loc. 14154). Bishops of country churches, still recognized as of higher rank than the presbyters, were not considered as important as those of cities. Bishops of the capitals of provinces were raised above others. They were called “metropolitans.” This system was already recognized by 325 (Schaff 2014, Loc. 14159). The churches with a distinctive apostolic heritage were the most respected. Because of their status as Roman capitals, Antioch, Alexandria, and Rome rose to prominence (Schaff 2014, Loc. 14165). Schaff next turns his attention to the rise of Rome and the Papacy.