Arius (Gr. Areiōs), the celebrated founder of Arianism, was a native of Libya, and is generally supposed to have been born shortly after the middle of the 3d century. About the year 306 A.D., Alexandria was thrown into confusion by the violence of its religious disputes, and in these Arius was largely concerned. At first, he took part with Meletius, Bishop of Lycopolis, in Upper Egypt, a man who was strenuously opposed to certain notions of discipline entertained by Peter, Bishop of Alexandria. He was excommunicated by Peter in consequence; but the latter dying soon after, Achillas, his successor, restored him to his office, and even advanced him to the dignity of a presbyter, 313. Arius was first brought into collision on a point of doctrine with his ecclesiastical superiors in 318. Alexander, the successor of Achillas, having in a public assembly of clergy, while speaking of the Trinity, said that it contained one single essence, or indivisible unity of substance, Arius alleged that such a conception was impossible to the human mind, and accused Alexander of Sabellianism—i.e. of destroying the distinction of persons. In maintaining his ground, Arius went beyond his first statement of the absolute distinctness of person between the Father and the Son; he maintained that the Son was not co-equal or co-eternal with the Father, but only the first and highest of all finite beings, created out of nothing by an act of God's free-will, and that he ought not to be ranked equal with the Father.
Arius was successful in securing the adherence of large numbers both of the clergy and laity in Egypt, Syria, and Asia Minor. In 321 a synod of bishops at Alexandria deposed and excommunicated him. To escape persecution, he retired to Palestine, whence he wrote a letter to his friend Eusebius, Bishop of Nicomedia. Eusebius warmly sympathised with him; wrote in his behalf to Paulinus, Bishop of Tyre, and others; absolved him from the Alexandrian synod's excommunication; and in 323 convened another synod in Bithynia, which pronounced favourably on Arius. While Arius was residing at Nicomedia, he wrote a theological work in verse and prose, called Thaleia, some fragments of which remain. The Thaleia is said to have been sung by the Arian neophytes, who thus kindled the passions of their adversaries, and increased the virulence of the contest. The comedians, who were pagans, took advantage of the occasion to ridicule the Christian religion in the theatres.
It now became impossible for the Emperor Constantine to remain neutral or indifferent, and in order, as he thought, to effect a final settlement of the question, he convoked the memorable Council of Nicaea, or Nice (see CREEDS), in Bithynia, 325. Three hundred and eighteen bishops from almost all parts of the Christian world, but especially from the East, were present, besides numbers of priests, deacons, and acolytes. Arius boldly expounded and defended his opinions. He declared in the most unambiguous manner that the Son of God was created out of nothing; that he had not always existed; that he was not immutable or impeccable; that it was through his free-will he remained good and holy; that if he had chosen, he could as easily have sinned as not; in a word, that he was a mere creature and work of the Deity. He further affirmed that the Son of God was not of the same substance with the Father; that he was not the 'Word' or 'Wisdom,' properly speaking; and that the Scriptures only attribute these names to him as they do to other created intelligences. These propositions were listened to with great calmness by the bishops, but the inferior clergy, or at least a majority of them, manifested the most violent opposition. Alexander, Bishop of Constantinople, was ably seconded by the young deacon, Athanasius (q.v.), the equal of Arius in eloquence, and in the power of his logic. It was principally by the reasonings of Athanasius that the Council was persuaded to define, in the most precise manner, the doctrine of the Godhead—viz. the absolute unity of the divine essence, and the absolute equality of the three persons. All the bishops subscribed it except two, who were banished, along with Arius, to Illyricum.
An imperial edict was now issued commanding the writings of the heresiarch to be burned, and threatening with capital punishment all who should be convicted of concealing them. But at Alexandria, the Arians continued in a state of open insurrection, and began to league themselves with other condemned sects. The great influence of Eusebius was also exerted on behalf of the exiled heretic, as well as that of Constantia, the sister of the emperor, who had herself embraced Arian tenets, and in 328 permission was granted to Arius to return from Illyricum. In 330 he had an interview with the emperor; and, in the confession of faith which he presented, he declared his belief that the Son was born of the Father before all ages, and that, as the 'Word,' he had made all things both in heaven and earth. The emperor was satisfied, and sent orders to Athanasius, now Bishop of Alexandria, to receive Arius into the communion of the church. This Athanasius refused to do, and a series of tumults was the consequence. Eusebius was greatly irritated. He called a synod of bishops at Tyre, in 335, which proceeded to depose Athanasius. The emperor was even prevailed on to remove him to Gaul. In the same year, another synod met at Jerusalem, which revoked the sentence of excommunication uttered against Arius and his friends. Still the majority of the Christians of Alexandria clung to the doctrines of Athanasius. Disappointed in his expectations, Arius, in 336, proceeded to Constantinople, where he presented the emperor with another apparently orthodox confession of faith; whereupon orders were issued to Alexander, Bishop of Constantinople, to administer to him the holy communion on the Sunday following. Before the ceremony, however, he died so suddenly that his disciples declared that he had been poisoned, while the orthodox devoutly affirmed that God had answered the prayers of Alexander.
The manners of Arius were graceful and modest; he was noted for even an ascetic abstinence, and the purity of his moral character was never challenged by a single enemy. He is said to have composed songs for sailors, millers, and travellers, in popular measures, for the purpose of spreading his peculiar tenets; but no traces of these survive.
After his death, his followers rallied round Eusebius, now Bishop of Constantinople (338), from whom they were styled Eusebians. Constans, who ruled the West after the death of Constantine (337), and Constantius in the East, made an essay towards reconciliation; but it failed at the synod of Sardis (347), where the occidental bishops gathered themselves round Athanasius in support of the Homoiousian doctrine (identity or sameness of substance), while in a separate council at Philippopolis, the oriental bishops asserted the Homoiousian doctrine (implying merely similarity of substance). Slight as might appear the verbal difference between the two parties, the bitterness of the controversy was intense, and pervaded almost all departments of public and private life. Constantius having gained dominion over the West, the Arian cause, which he favoured, triumphed at the synod of Arelate or Arles (353) and at that of Milan (355). These victories, however, were more apparent than real. The Nicene doctrine had still strong support on its side, and was strictly maintained by the banished Athanasius and his friends, while the Antiniceans, soon after their triumph, were divided into at least three parties. The old Arians, also styled Anomoeans, or Heterousians, asserted, in the boldest style, their doctrine of 'distinct substances.' The semi-Arians (a large majority in the Eastern Church) maintained the Homoiousian doctrine of similar substances. A third party held the same doctrine with some qualification. Morally, the victory was leaning to the side of the Niceneans. Julian the Apostate (361-3), in his hatred of the Christian religion, left all parties at liberty to contend as they pleased with one another, so that they did not interfere with his plans. Jovianus and his followers in the West, Valentinianus I., Gratianus, and Valentinianus II., extended full toleration to both parties. Arianism at last was virtually suppressed in the Roman empire, under Theodosius in the East (379-95), and Valentinianus II. in the West. Among the German nations, however, it continued to spread through missionary efforts. Bishop Ulfilas, the translator of the Bible into the Meso-Gothic language, had been the means of converting the West Goths to Arian Christianity as early as 348; and they adhered to it until the synod of Toledo in 589. The East Goths, Vandals, Burgundians, the Suevi in Spain, and the Lombards also adopted Arianism; but in all these instances the Nicene doctrine ultimately prevailed, most slowly among the Lombards, who retained the Arian creed until 662. The Arian controversy was revived in England for a time by the writings of the learned Dr Samuel Clarke (1675-1729), and also by Whiston (1667-1752). The denial of 'the eternal sonship' was broached in the Wesleyan Methodist Society by Dr Adam Clarke (1762-1832) and a few followers; but it was soon suppressed by the Conference. The greatest English writer who held distinctly Arian or semi-Arian views was Milton. Arianism has been superseded by Unitarianism. See Kölling's Geschichte der Arianischen Häresis (2 vols. Gütersl. 1875-83).