The eventual emergence of Christianity as the predominant religion in the Roman empire was due to a variety of advantages which it held over rival worships. The Christians provided themselves with an organization surpassing that of all the other private religions. When the early Christian congregation broke away from the parent Jewish church, it lost all the advantages of membership in a well-regulated society. The first Christian communities were isolated cells under a rudimentary administration of elder members. But, in the first century of their existence, they instituted a well-organized body of clergy possessing wide powers of discipline over the laity. By the time of M. Aurelius, the clerical hierarchy was complete in all essentials. Still more important was the creation of a unique system of intercommunication between the several Christian communities. In the first century AD, the only means of keeping in touch between the individual churches was by irregular visits or occasional epistles from authoritative leaders like Peter and Paul. In the course of the second century, the churches evolved a system of regular correspondence by representatives of neighboring congregations. In the early years of the third century, a critical step forward was taken when, in one Roman province after another, the bishop of the “metropolis,” or principle town, proceeded to convene regular synods of bishops from all the lesser cities, on the pattern of the secular concilia. At these conferences, provision was made for mutual financial support – a form of insurance, whose value was often proved in times of persecution – and a uniform creed was formulated. Whatever the ultimate results of insistence on orthodox belief, its immediate effect was to check the spread of unauthorized creeds which, at this stage of church history, might have caused serious confusion of thought. Under the reign of Constantine, the provincial synods were supplemented by “ecumenical” congresses attended by prelates from more than one province. In 314, a representative gathering of bishops from the western churches assembled at Arelate. In 325, a gathering of higher clergy from every part of the empire was held at Nicea in Bithynia, whose work survives in the “Nicene” creed. By 330, the administrative framework of the church universal was complete, in all essentials.
From: A History of Rome Down to the Reign of Constantine by M. Cary; 2nd edition (London: Macmillan & Co., Ltd., 1954), pp. 762-763. First edition published in 1935.