Christians recognise the distinctness of God the Father, God the Son, and God the Holy Spirit - or God the Holy Ghost, to use the language of the Authorised Version of the Bible and the Book of Common Prayer - but they nonetheless believe that there is one God. It is therefore necessary for them to clarify the relationship between God the Father, God the Son, and God the Holy Spirit, and to do so in a way which explains how despite their distinctness they are a single God. The orthodox account, usually referred to as the doctrine of the Trinity, is that God the Father, God the Son, and God the Holy Spirit are three 'persons' but one 'substance'. Whatever the precise import of this doctrine, it is clear that the theologians who formulated it early in the fourth century intended to emphasise that the three 'persons' of the Trinity have always existed together, the existence of the one being inseparable from the existence of the others, and that they have equal status. As the Athanasian Creed puts it, 'the Godhead of the Father, of the Son, and of the Holy Ghost, is all one: the Glory equal, the Majesty co-eternal. Such as the Father is, such is the Son:and such is the Holy Ghost'.
Arianism, i.e., the teaching of Arius, conflicts with the orthodox position by denying that God the Father, God the Son, and God the Holy Spirit are the same substance and are of equal status. Arius held that God the Son was created by God the Father, who through God the Son created God the Holy Spirit - although he insisted that these acts of creation took place before time began and so it was not possible to say that there was ever a time when God the Son or God the Holy Spirit did not exist. Arius thus denied that the three 'persons' in the Godhead are the same substance (ὁμοούσιος, homoousios): he said they are of a similar substance (ὁμοιούσιος, homoiousios).
Arius (c250-336) originally came from Libya and was a priest in Alexandria in Egypt. Although his teaching was condemned by Alexander, the bishop of Alexandria, it was widely accepted in the eastern half of the Roman Empire, and disagreements between Arians and orthodox Christians became so bitter that the emperor himself, the recently converted Constantine I (reigned 306-337) felt obliged to intervene. He first wrote to Arius and Alexander suggesting that they might simply 'agree to differ', but this plea fell on deaf ears, and in 325 he summoned the first ecumenical council of bishops at Nicaea in northwest Asia Minor to decide the issue. The council condemned Arius' teaching as a heresy and excommunicated Arius himself as a heretic: the Nicene Creed, which was drawn up by the council, was framed in a way which made it impossible for Arians to assent to it - for example, Jesus is referred to as 'the only begotten Son of God ... begotten, not made, being of one substance with the Father'. (The Athanasian Creed, which was formulated later by Athanasius, a Doctor of the Church, Alexander's successor as bishop of Alexandria and also implacably opposed to Arius, is even more emphatic in its statement of the orthodox doctrine of the Trinity.) Notwithstanding its condemnation by the Council of Nicaea Arianism continued to flourish - in the fourth century two Roman emperors (Constantius II (reigned 324-361) and Valens (reigned 364-378)) were Arians - and among the German tribes in the north it was still widely accepted in the seventh century. Arius' status as a heretic was rescinded in 335, shortly before his death, but he was again excommunicated, this time posthumously, at the first Council of Constantinople in 381.
The word 'Arianism' is pronounced with the stress on the first syllable AIR-i-ern-ism, IPA: /'ɛə rɪ ə ,nɪ zəm/.