Arianism was a Christian heresy first proposed early in the 4th century by the Alexandrian presbyter Arius. It affirmed that Christ is not truly divine but a created being. The fundamental premise of Arius was the uniqueness of God, who is alone self-existent and immutable. The Son, who is not self-existent, cannot be God.
An ascetical, moral leader of a Christian community in the area of Alexandria, Arius attracted a large following through a message integrating Neoplatonism, which accented the absolute oneness of the divinity as the highest perfection, with a literal, rationalist approach to the New Testament texts. Christ was viewed as the most perfect creature in the material world, whose moral integrity led him to be "adopted" by God as a son but who nevertheless remained a secondary deity, or Logos substantially unlike the eternal, uncreated Father and subordinate to his will. Because the Godhead is unique, it cannot be shared or communicated so that the Son cannot be God. Because the Godhead is immutable, the Son, who is mutable (being represented in the Gospels as subject to growth and change) cannot be God. The Son must, therefore, be deemed a creature who has been called into existence out of nothing and has had a beginning. Moreover, the Son can have no direct knowledge of the Father since the Son is finite and of a different order of existence. This thesis was publicized ~323 through the poetic verse of his major work, Thalia (Banquet), and was widely spread by the tactic of popular songs written for laborers and travelers.
According to its opponents, especially Athanasius (http://www.ntcanon.org/Athanasius.shtml), Arius' teaching reduced the Son to a demigod, reintroduced polytheism (since the worship of the Son was not abandoned), and undermined the Christian concept of redemption since only Christ who was truly God could redeem the world. From the outset, the controversy between both parties took place upon the common basis of the Neoplatonic concept of ousia ("substance" or "stuff"), which was foreign to the New Testament itself.
Following and exchange of condemnations (323-324) between the Arians and various gatherings of clergy in Egypt, Palestine, and Syria, Constantine, eager for unity and peace, sent emissaries to mediate the conflict. This effort failed, and he summoned the Council of Nicaea (the First Ecumenical Council) in May 325, to settle what he termed "a fight over trifling and foolish verbal differences". The bishops issued a creed to safeguard orthodox Christian belief. This creed states that the Son is homoousion to Patri (of one substance with the Father), thus declaring him to be all that the Father is: he is completely divine. When Arius refused to sign the creed, the bishops declared him a heretic and exiled him and the Arian leaders. This seemed to end the controversy, but it was only the beginning of a long-protracted dispute.
Although the Arian leaders were exiled, they tried by intrigue to return to their churches and sees and to banish their enemies. They were partly successful. Influential support from colleagues in Asia Minor and from Constantia, the Emperor's daughter, succeeded in effecting Arius' return from exile and his readmission into the church after consenting to a compromise formula, despite the opposition from Athanasius (http://www.ntcanon.org/Athanasius.shtml). Shortly before he was to be reconciled, however, Arius collapsed and died while walking through the streets of Constantinople in 336.
When Constantine died in 337, Constans became emperor in the West and Constantius II became emperor in the East. The former was sympathetic to the orthodox Christians and the latter to the Arians. At a council held at Antioch (341), an affirmation of faith that omitted the homoousion clause was issued. Another council was held at Sardica in 342, but little was achieved by either council.
In 350 Constantius II became sole ruler of the empire, and other his leadership the Nicene party (orthodox Christians) was largely crushed. The extreme Arians then declared that the Son was anomoios (unlike) the Father. These Anomoeans succeeded in having their views endorsed at Sirmium in 357, but their extremism stimulated the moderates, who asserted that the Son was homoiousios (of similar substance) with the Father, and conservatives, who asserted that the Son was homoios (like) the Father. Constantius at first supported the Homoiousians but soon transferred his support to the Homoenas, led by Acacius. Their views were approved in 360 at Constantinople, where all previous creeds were rejected, the term ousia ("substance" or "stuff") was repudiated, and a statement of faith was issued stating that the Son was "like the Father who begot him".
After Constantius' death in 361, the orthodox Christian majority in the West consolidated its position. The Arian persecution conducted by Emperor Valens (364-378) in the East and the success of the teaching of Basil the Great of Caesarea, Gregory of Nyssa, and Gregory of Nazianzus led the Homoiousian majority in the East to realize its fundamental agreement with the Nicene party. When the emperors Gratian (367-383) and Theodosius I (379-395) took up the defense of orthodoxy, Arianism collapsed. In 381 the Second Ecumenical Council met at Constantinople. Arianism was proscribed and the Nicene Creed was approved.
Although this ended the heresy in the empire, Arianism continued among some of the Germanic tribes to the end of the 7th century. In modern times some Unitarians are virtually Arians in that they are unwilling either to reduce Christ to a mere human being or to attribute to him a divine nature identical with that of the Father. The Christology of the Jehovah's Witnesses is also a form of Arianism; they regard Arius as a forerunner of Charles Taze Russell, the founder of their movement.
The above was taken from the Encyclopędia Britannica.