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Homoiousianism (from the Greek: ὁμοιούσιος from ὅμοιος, hómoios, "similar" and οὐσία, ousía, "essence, being") was a 4th century AD movement which arose in the early period of the Christian religion out of a wing of Arianism. It was an attempt to reconcile the seemingly irreconcilable views of the pro-Nicene homoousians, who believed that God the Father and Jesus his son were identical (ὁμός, homós) in substance, with the "neo-Arian" position that God the Father is "incomparable" and therefore the Son of God can not be described in any sense as "equal in substance or attributes" but only "like" (ὅμοιος, hómoios) the Father in some suborbinate sense of the term.
Homoiousia /ˌhɒmoɪˈaʊsiə/ HOM-oy-OW-see-ə is the theological doctrine that Jesus the Son of God and God the Father are of similar (ὁμοιο- homoio- or homeo-) but not the same substance, a position held by the Semi-Arians in the 4th century. It contrasts with the homoousia of orthodox Trinitarianism and the heteroousia of Arianism.
During the period of the development of Christian doctrine which ran from 360 to 380 AD, the controversy between Arianism and what would eventually come to be defined as orthodoxy provoked an enormous burgeoning of new movements, sects and doctrines which came into existence in the attempt to stabilize and consolidate a unique and universal position on complex and subtle theological questions. One of the main questions concerned the nature of God and the nature of his relationship with his Son, Jesus Christ. This controversy was called the "trinitarian controversy" because it involved solving the riddle of how it was possible that God could be three (God the Father, His Son Jesus the Word, and the Holy Spirit) and yet One at the same time. The dominant position among Christian theologians at this point in history was the doctrine of homoousianism, according to which Father and Son were identical in substance and in attributes and that any deviations from this orthodoxy were to be considered heresy. The Homoians, however, had a powerful ally on their side in the person of Emperor Constantius II.
The Homoiousians took a moderate stance between that of the Homoousians, and heteroousian such as Aetius and Eunomius. At a council in 358 at Sirmium, at the height of the movement's influence, the claim was made that the Son is "like [the Father] in all [respects]" (ὅμοιον κατὰ πάντα, hómoion katà pánta), while the use of οὐσία (ousía) or any of its compounds in theological discussion was strongly criticized but not abandoned, and the Anomoeans were anathematized. This compromise solution, which was satisfying to both the Homoians and the Homoiousians, deliberately set out to alienate the more extreme Neo-Arians. It was successful in this intent but it remained as illegitimate in the eyes of the pro-Nicenes as ever and Basil of Ancrya declared that "that which is like can never be the same as that to which it is like". On the other side, Constantius was becoming somewhat hostile to the influence of all of the new movements which had sprung up after the Nicene council. The result was that the Homoiousians disappeared from the stage of history and the struggle to define Church dogma became a two-sided battle between the Homoousians and the Homoians.
- Steenberg, M.C. A World Full of Arians: A Study of the Arian debate and the Trinitarian Controversy from AD 360-380. 2000.