Such were the things accomplished by Helena, while, under a Christian prince, the world had both attained to liberty, and possessed in him an exemplar of faith. But a far more dreadful danger than all that had preceded fell upon all the churches from that state of tranquillity. For then the Arian heresy burst forth, and disturbed the whole world by the error which it instilled. For by means of the two Ariuses, who were the most active originators of this unfaithfulness, the emperor himself was led astray; and while he seemed to himself to fulfill a religious duty, he proceeded to a violent exercise of persecution.
The bishops were driven into exile: cruelty was exerted against the clerics; and even the laity were punished, who had separated from the communion of the Arians. Now, the doctrines which the Arians proclaimed were of the following nature,— that God the Father had begotten his Son for the purpose of creating the world; and that, by his power, he had made out of nothing into a new and second substance, a new and second God; and that there was a time when the Son had no existence.
To meet this evil, a synod was convened from the whole world to meet at Nicæa. Three hundred and eighteen bishops were there assembled: the faith was fully set forth in writing; the Arian heresy was condemned; and the emperor confirmed the whole by an imperial decree. The Arians, then, not daring to make any further attempt against the orthodox faith, mixed themselves among the churches, as if they acquiesced in the conclusions which had been reached, and did not hold any different opinions. There remained, however, in their hearts, a deep-seated hatred against the Catholics, and they assailed, with suborned accusers and trumped-up charges, those with whom they could not contend in argument on matters of faith.
Source: Sacred History (New Advent)