Semi-Arianism is a name that has been used for identifying a position that held to a version of the Nicene Creed that omitted the formula “of One Substance”. This position was taken after the First Ecumenical Council in 325 condemned Arianism as heresy by those Christians who kept a Trinitarian view but in practice took a compromised stand whereby they remained in communion with Arians without adopting Arianism itself. The group, led by Bishop Basil of Ancyra, advocated the use of the term Homoiousios over that of homoousios, an iota of difference that denied the consubstantiality of Christ. During this time the Arians, who took the original position of Arius that Jesus of Nazareth was of a different nature from and in no way like that of God the Father, were known as Anomoeans, from anomoios, unlike.
After the First Ecumenical Council’s decision on Arianism, strong efforts continued to be exerted by some bishops, who were to be termed Semi-Arians, who had denied being Arians, to find a compromise position between the use of homoousios (of One Substance) in the Nicene Creed and the Arian position that Jesus Christ was subordinate to, and unlike, God the Father. These bishops, who were not firm Arians and supported the Nicene Creed at the First Ecumenical Council at Nicea yet were not fully Orthodox Trinitarian themselves, received repentant Arians to communion. The semi-arian bishops were among those who had agreed to the deposition of St Athanasius of Alexandria at Tyre in 335.
In the decades following Nicea, many who opposed the use of homoousios as non-biblical, brought forward various formula, some of which even the semi-arian bishops opposed, that centered around use of the word homoiousios (similar substance) as alternates for the Nicene Creed. These bishops attempted to get the ear of Emperor Constantius II who favored the semi-arians and despised Athanasius of Alexandria, champion of Orthodoxy.
The dispute continued thorough a number of local councils, usually dominated by non-Orthodox clergy, where various versions of creeds were put forward by the semi-arians that avoided the use of homoousios. These included an Arian Council of Antioch of 341 and the second Council Sirmium in 351. At the third Council of Sirmium in 357, both homoiousios and homoousios were thrown out of a confession, later to be known as the Blasphemy of Sirmium. The Council of Ancyra in 358, chaired by Basil of Ancyra, used the term homoiousios, while the fourth Council of Sirmium used the term homoios, by which the Son is “like” the Father. In 359, homoios was encouraged by Constantius II at the Council of Rimini and the Council of Seleucia at which the semi-arians maintained the use of homoiousios, and condemned the Anomoean party of Arians. At a Council of Constantinople in 361, the use of homoios was advocated by those that became known as Homoeans, stating that the Son was “like the Father who begot him”, and rejected the use of ousia (substance). With the ascension to the imperial throne of Julian the Apostate in 361, after the death of Constantius II, the original position of Arius re-appeared supported by the Anomoeans at a Arian council in Antioch.
After the death of the Apostate in 363, the leaders of the Nicene orthodoxy, Athanasius, Basil of Caesarea, Gregory of Nyssa, and Gregory of Nazianus, with a re-vigored Nicene party in the West, consolidated their position affirming the use of homoousios. In 380, Emperor Theodosius I, who was a stanch Trinitarian, outlawed Arianism when he ascended the throne. He also, convened the Second Ecumenical Council in Constantinople in 381 that upheld an amended version of the Nicene Creed: the Nicene-Constantinopolitan Creed.
By 383, the Nicene-Constantinopolitan Creed stood alone without being further contested by further statements of faith from the Arians or Semi-Arians.