The Arian Philosophy
The basis of Arianism lies in the premise that the Christian god is unique and never-changing. As the son of God, Jesus did change (according to the Gospels, he was born, grew physically, and died); therefore, Arius concluded, he could not also be God and was not truly divine. Arius posited that God created Jesus and that God the Father, Jesus Christ the Son, and the Holy Spirit, or Ghost, were materially three separate entities.
Opposition to Arianism
By the fourth century, the idea that the Father, Son and Holy Ghost were materially the same (three parts of the Holy Trinity), and that all three were divine, had become fairly well-established as mainstream Christian thought. Those who professed this viewpoint were known as Trinitarians, and the idea of all three aspects of the Trinity sharing the same substance was known as homoousion.
The alternate interpretation of Arius caused a controversy in and around Alexandria. His most stalwart opponent was Bishop Alexander of Alexandria and Alexander's deacon Athanasius (who later became bishop in his own right). They argued that the Arian view reduced Jesus to a demigod, and that, since Arian Christians still worshipped Jesus, this would make them polytheists. Furthermore, if Jesus and God were not one and the same, the idea of redemption would be undermined, for only God could reconcile the sinner to the divine, but it was Jesus who had died on the cross to do just that.
At the Council of Nicea in 325, bishops from all over the Roman Empire met to discuss the Arian problem under the leadership of Emperor Constantine, the first Roman Emperor to profess Christianity. Arius and Eusebius, Bishop of Nicomedia, argued against homoousion. But, following Constantine's inclination, the council decided that the Trinitarian view was the Right Way, and set forth a statement, the Nicene Creed, for believers to express what was now officially the orthodox view. Arius and some Arian leaders were exiled, and his teachings were condemned.
The judgment of the Council of Nicea might have put an end to the Arian heresy if Arius had not had allies who worked to get him and his fellow leaders into positions of importance. In time, Constantine reversed his opinion on the heresy and reinstated some bishops to their offices. Arius himself was on the verge of getting reinstated into the Church when he died suddenly in 336 C.E. His fellow Arians claimed he'd been poisoned; his opponents pointed to his death as divine judgment. In any case, the death of Arius did not mean the end of Arianism.
Arianism on the Rise
The Arian heresy continued to spread among citizens of Rome, and several emperors took up the Arian philosophy. From 337 to 350 Constantius II ruled as emperor of Eastern Rome and was sympathetic to Arian Christians. When he became sole ruler of a re-joined empire in 350, the Trinitarians were suppressed. There were even persecutions of Trinitarian Christians under Emperor Valens (eastern emperor from 364 to 378).
During the reign of Constantius II, there were several minor councils at which the Arians were able to get their variety of Christianity officially endorsed. But these days of Arian dominance could not last.
The Pendulum Swings Back
Enjoying the upper hand at last, some Arians pushed a little too far, prompting moderate Christians to take action. Rational believers asserted a less extreme interpretation of the nature of God and Jesus, and Constantius II began to listen. In time, the emperor began to consider the Trinitarian view once more. In 360, he called another council, this one at Constantinople. A new statement of faith was issued: the Homoian Creed, and all other creeds were rejected.
The next few decades were somewhat tumultuous for Arian and Orthodox Christians alike. Julian the Apostate tried to get "paganism" instituted as the official religion of the empire, but his efforts ultimately backfired. When Emperors Gratian and Theodosius I championed the cause of Orthodoxy, Arianism virtually crumbled. One last council, this time an Ecumenical Council at Constantinople, put the final nail in the coffin of Arianism -- at least as far as the Roman Empire was concerned.
Arian Barbarians Live On
Although Arianism was virtually wiped out in the Roman Empire, the philosophy continued among barbarian tribes for several more centuries, and the fact that these people espoused what the Romans considered a heresy would cause complications among the growing kingdoms of the fifth and sixth centuries. Eventually, the tribes either converted to orthodoxy or died out by the end of the sixth century.