ARIANISM, a theological system in the early Christian Church, named after Arius, a presbyter at Alexandria. In opposition to his bishop Alexander, Arius asserted that there was a time when the Son was not coequal, since the Father who begot must be before the Son who was begotten, and the latter therefore could not be eternal. As many prominent bishops sided with Arius, synods were called on both sides, and the most acute intellects of the church discussed the question. The general council of Nice (325), attended by 300 bishops, condemned Arius and declared the Son to be consubstantial with the Father; but Arius nevertheless gained the favor of Constantine and won many new adherents. After his death (336) the movement spread more rapidly than before. When Constantine died in 337, the empire was divided among his three sons, two of whom, Constantine and Constans in the West, accepted the Nicene creed, while Constantius in the East was a decided favorer of Arianism. An anti-Nicene council at Antioch (341), consisting of 90 bishops, issued decrees on the ground of which Athanasius, who in 338 had returned from exile to his diocese, was again deposed. In the West, on the contrary, a synod at Rome in 343 declared Athanasius innocent of the charges preferred against him and the authors of his exile heretics. In order to put an end to this conflict, Constantius and Constans (Constantine had died in 340) convoked the general synod of Sardica in Lower Mœsia in 343 or 344 (not, as has heretofore been generally assumed, in 347). The Arians, having a minority of the 176 bishops present, held a council of their own, at first in the imperial palace in Sardica, and subsequently at Philippopolis. Each party anathematized the other; but the Nicæans triumphed. Constantius so far yielded to the remonstrances of Constans as to allow the return of Athanasius (349); but when he became soon after sole ruler of the empire, his influence at the synods of Arles (353) and Milan (355) secured the condemnation of Athanasius and the adoption of Arian decrees. Pope Liberius and several bishops, among them Athanasius, were banished, and Arianism was completely successful. The sect now became divided into strict and moderate Arians. Eusebius of Cæsarea declared the Son to be homoiousios or similar in substance to the Father, and his followers were called Homoiousians or Semi-Arians. In opposition to him, Eusebius of Nicomedia showed himself an uncompromising Arian. When the emperor attempted to enforce the Arian resolutions of Milan in the place of those of Nice, the strict Arians, under the leadership of Aëtius, deacon at Antioch, and Eunomius, bishop of Cyzicus in Mysia, attacked the Semi-Arians as well as the Nicene doctrine as illogical, and developed in opposition to it a strict subordinationism. The reputation of Eunomius in his party was so great, that their original name of Aëtians gradually gave way to that of Eunomians. They were also called Anomœans, Heterousiasts, and Exoucontians, as they maintained that the Son was dissimilar to God (ἀνόμοιος), of different essence (ἑτέρας οὐσίας), and created out of nothing (ἔξ οὐκ ὄντων). Several synods were held for the purpose of healing these divisions. At the second great synod of Sirmium (357) a confession of faith was adopted, to which not only the strict Arians, but even the Nicene bishops, including their leader Osius of Corduba, subscribed. But the confusion became greater than ever. An Arian synod at Antioch (358) condemned, while a Semi- Arian synod at Ancyra (358) approved the expression homoiousios. At the third synod of Sirmium (359) Pope Liberius subscribed to a Semi-Arian declaration in order to obtain permission to return from Constantinople to Rome. The Semi-Arians seemed to be in the ascendancy; the emperor is said to have exiled no fewer than 70 strict Arians, and Bishop Marcus of Arethusa was instructed to draw up a new confession of faith, the fourth Sirmian formula, which avoided the word ousias and affirmed that the Son was similar in everything to the Father. In order to reunite the whole church on this platform, Constantius wished to call an œcumenical council; but the influence of the Arians caused the convocation of two synods, an eastern one at Seleucia, and a western at Rimini. At the former there were present 105 Semi-Arians, 40 strict Arians, and 10 Nicæans; at Rimini the Nicæans had a majority. Both synods condemned the strict Arians, who however succeeded in regaining the favor of the emperor. Threats induced nearly all the bishops of both synods to subscribe to a strict Arian creed, although the most offensive party expressions were studiously avoided, and even a few of the uncompromising leaders of the party, as Aëtius, sent into exile. Thus Arianism was looked upon as the official creed of the majority of Christian bishops. But its ascendancy was of short duration. On the death of Constantius (361) and the accession of Julian the Apostate, the bishops of all parties were allowed to return to their sees, and soon the Nicene party reëstablished themselves in Egypt under Athanasius, and in Gaul, Spain, and Greece. Pope Liberius ratified the anti-Arian resolutions passed in 362 by the synod held in Alexandria, and soon the Nicene creed was predominant throughout the western countries. In the East, Arianism found a zealous supporter in the emperor Valens (364-378), and the violent measures which were adopted against both the Nicæans and the Semi-Arians induced a portion of the latter (366) to submit to the Nicene creed. With the death of Valens (378) Arianism began to decline. The emperor Gratian issued an edict of toleration (378), which allowed the exiled bishops to return and greatly strengthened the Nicene party. In 379 Gratian shared the empire with Theodosius, who the next year issued an edict threatening all heretics with the heaviest penalties, and as soon as he arrived in Constantinople took from the Arians all their churches. In 381 he convened the second œcumenical council at Constantinople, which anathematized the Arians. In another synod held at Constantinople in 383, Eunomius presented his confession of faith, which is still extant. As the Arian leaders refused to submit, still more rigorous decrees were issued, to which they appear to have soon succumbed, for the last trace of them in the eastern empire ceased under the reign of Arcadius, the son of Theodosius. In Italy the empress Justina, while regent for her minor son Valentinian II., favored the Arians; but Ambrose, the great bishop of Milan, successfully thwarted her plans, and at the synod of Aquileia (September, 381) caused the Arians to be anathematized and deposed. Moreover, the reign of Justina was too short to be of real service to the dying sect.—Crushed out in the Roman empire, Arianism for several centuries remained the religion of the Germanic tribes. The Ostrogoths professed Arianism, but without persecuting the Catholic church, until their power was lost in 553. The Visigoths were more intolerant, but in 589, by order of their king Reccared, they joined the Catholic church at the council of Toledo. The Arian Vandals, after conquering Africa under Genseric (429), began a most cruel persecution of the Catholics, which did not cease until the destruction of their empire by Belisarius (534). The Suevi in Spain adopted the Arian form of Christianity toward the middle of the 5th century; about 558 they joined the Roman communion. The Burgundians, who had come to Gaul as pagans (407), appear in 450 as Arians. The Catholic church became predominant among them under King Sigismund (517), whom Bishop Avitus of Vienne had won over to the orthodox creed. The last refuge of Arianism was with the Lombards, who entered Italy as Arians in 568. The Catholic church gained a footing among them through the wife of King Autharis, the Bavarian princess Theodelinda; and under her second husband Agilulph and her son Adelwald the Catholics obtained possession of most of the churches. A reaction followed when an Arian ascended the throne; but he was unable to suppress Catholicism, and for a time every important town had a Catholic and an Arian bishop. Under Liutprand (died 744) Arianism as a sect became extinct. As a theological opinion, however, it often reappeared, and after the reformation of the 16th century was regarded by more than one religious denomination as the true doctrine of the person of Christ. In the church of England Arian views found learned champions in Professor Whiston and Dr. Samuel Clarke.—The works of the Arian writers are mostly lost; we still possess, however, the writings of Eusebius of Cæsarea, who ranks among the ablest defenders of the ancient system, and fragments of the church history of Philostorgius. Histories of Arianism have been written by Maimbourg (Histoire de l'Arianisme, Paris, 1682) and J. A. Stark (Versuch einer Geschichte des Arianismus, Berlin, 1783); but the best source of information on the controversial aspect of the question is Baur's Geschichte der christlichen Dreieinigkeit (Tübingen, 1841-'3); while the history of the sect is nowhere treated of so fully as in Hefele'e Conciliengeschichte (vols. i. and ii., Tübingen, 1855). See also Revillout, De l'Arianisme des peuples germaniques (Paris, 1850).