Eunomianism, a phase of extreme Arianism prevalent amongst a section of Eastern churchmen from about 350 until 381; as a sect it is not heard of after the middle of the fifth century. The teaching of Arius was condemned by the Council of Nicaea, and the word homoousion adopted as the touchstone of orthodoxy. The subsequent history of the Arian heresy is the history of the endeavors of arianizing sympathizers to get rid of the obnoxious word. The diplomacy of court intriguers forms the dark background against which stand out Eusebians and Semi-Arians. Imperial influence had been all-powerful too long in the official religion to allow imperial ingerence in church affairs to cease with the imperial change of attitude towards Christianity. That influence was exercised through the court prelates tinged with the fundamental rationalism underlying Arianism. They skillfully avoided the real issue, represented the whole affair as merely a question of the propriety of using particular terms, and for a time deluded those who were unfamiliar with the metaphysics of the question. St. Athanasius was represented as a political fire-brand whose watchword was homoousion. The Emperor Constantius (337-361), to his great personal annoyance, was obliged to allow Athanasius to return from his second exile (339-346) to Alexandria (October 31, 346). The lull which seemed to follow the return of Athanasius was due to the political circumstances arising out of the disastrous Persian War and the civil war against Maxentius; and it was not until the victory of Mount Seleucus (August 13, 353) that the emperor's hands were freed.
In the meantime a new and more defiant Arian school was arising, impatient of diplomacy, and less pliant to imperial dictation. It frankly returned to the fullest expression of the errors of Arius, and sought to defend it on the rationalizing basis of Aristotelean dialectics. The history of the new school coincides with the life-history of Aetius and Eunomius. Aetius, its founder, successively a goldsmith, physician, and grammarian, turned his attention to theology under Arian influences at Antioch and Alexandria. Aristotle's categories henceforth formed the limits of his knowledge, and the abuse of the syllogism his principal weapon. Ordained deacon at Antioch in 350, he was deposed by Leontius and sought refuge at Alexandria, where he found a disciple in Eunomius. Radical and uncompromising in their heretical teaching, they asserted that in substance and in all else the Son is unlike the Father: ἀνόμοιος, "unlike", became their watchword as against the ὁμοούσιος of the Orthodox, the ὁμοιούσιος of the Semi-Arians, and the later ὅμοιος (homoios) of the Acacians. Hence the Arian extremists became known as Aetians, and later as Eunomians and Anomoeans. Their doctrines were received favorably by Eudoxius of Antioch and the Synod of Antioch in 358; but the formulation of their tenets produced a reaction, and in the same year they were condemned by the Semi-Arians a Ancyra and at the Third Synod of Sirmium, and the leaders were exiled for a short time to Pepuza. They reappeared, however, at the Semi-Arian Synod of Seleucia (September, 359), where Acacius of Caesarea rejected the ἀνόμοιος and the triumph of the Homoeans led to the exile of Aetius to Mopsuestia in Cilicia and later to Amblada in Pisidia. After 360 the Anomoean Arians ceased to be formidable. Julian the Apostate (361-363) allowed Aetius to return; he was rehabilitated in an Arian synod, and died c. 370. Meanwhile Eunomius, supported by his friend Eudoxius, transferred from Antioch to Constantinople (January, 360), became Bishop of the Orthodox See of Cyzicus in Mysia. His flock appealed to Constantius, who obliged Eudoxius to take action against him. Deposed in his absence and banished, Eunomius founded a sect of his own, ordained and consecrated some of his followers. Julian recalled both Aetius and Eunomius, who acquired considerable importance in Constantinople. The Synod of Antioch, 362, explicitly set forth the Anomoean doctrine that "the Son is in all things unlike [κατὰ πάντα ἀνόμοιος] the Father, as well in will as in substance". The death of Eudoxius in 370 marks the beginning of the end of Eunomianism. The sectaries were excluded from the benefit of Gratian's edict of toleration (end of 378), were directly condemned by the Council of Constantinople (381), and were the objects of special repressive measures in addition to those directed against Arians and heretics in general. Moreover, disruptive forces were at work within the sect. Eunomius died about 395, and for all practical purposes the sect may be said to have died with him.