This reign is no less remarkable for the religious quarrel which then divided the Christians, which set church against church and bishop against bishop, as soon as they lost that great bond of union, the fear of the pagans.  Jesus of Nazareth was acknowledged by Constantine as a divine person; and, in the attempt then made by the Alexandrians to arrive at a more exact definition of his nature, while the emperor was willing to be guided by the bishops in his theological opinions, he was able to instruct them all in the more valuable lessons of mutual toleration and forbearance.  The followers of early religions held different opinions, but distinguished themselves apart only by outward modes of worship, such as by sacrifices among the Greeks and Romans, and among the Jews and Egyptians by circumcision, and abstinence from certain meats.  When Jesus of Nazareth introduced his spiritual religion of repentance and amendment of life, he taught that the test by which his disciples wrere to be known was their love to one another.  After his death, however, the Christians gave more importance to opinions in religion, and towards the end of the third century they proposed to distinguish their fellow-worshippers in a mode hitherto unknown to the world, namely, by the profession of belief in certain opinions; for as yet there was no difference in their belief of historic facts.  This gave rise to numerous metaphysical discussions, particularly among the more speculative and mystical.

At about this time the chief controversy was as to whether Christ was of the same, or of similar substance with God the Father, this being the dispute which divided Christendom for centuries.  This dispute and others not quite so metaphysical were brought to the ears of the emperor by Alexander, Bishop of Alexandria, and Arius, the presbyter.  The bishop had been enquiring into the belief of the presbyter, and the latter had argued against his superior and against the doctrine of the consubstantiality of the Father and the Son.  The emperor’s letter to the theologians, in this first ecclesiastical quarrel that was ever brought before a Christian monarch, is addressed to Alexander and Arius, and he therein tells them that they are raising useless questions, which it is not necessary to settle, and which, though a good exercise for the understanding, only breed ill-will, and should be kept by each man in his own breast.  He regrets the religious madness which has seized all Egypt; and lastly he orders the bishop not to question the priest as to his belief, and orders the priest, if questioned, not to return an answer.  But this wise letter had no weight with the Alexandrian divines.  The quarrel gained in importance from being noticed by the emperor; the civil government of the country was clogged; and Constantine, after having once interfered, was persuaded to call a council of bishops to settle the Christian faith for the future.  Nicaea in Bithynia was chosen as the spot most convenient for Eastern Christendom to meet in; and two hundred and fifty bishops, followed by crowds of priests, there met in council from Greece, Thrace, Asia Minor, Syria, Arabia, Egypt, and Libya, with one or two from Western Europe.