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Homoousian (from the Greek ομού meaning common and ουσία meaning essence or being) is a technical theological term used in discussion of the Christian understanding of God as Trinity. The Nicene Creed describes Jesus as being homoousian with the Father - that is, they are of the same substance and are equally God. The term, officially adopted by the First Council of Nicaea, was intended to add clarity to the relationship between Christ and God the Father within the Godhead.

The Nicaean Creed is the official doctrine of the Roman Catholic Church, Eastern Orthodox Church, Oriental Orthodox, Anglican Church, and most mainline Protestant churches (e.g. the Lutheran Church ) with regard to the ontological status of the three parts of the Trinity: Father, Son and Holy Spirit.

Some theologians preferred the use of the term homoiousios (Greek for "of like substance") in order to emphasize distinctions among the three persons in the Godhead, but the term homoousios became a consistent mark of Nicene orthodoxy in both East and West. According to this doctrine, Jesus Christ is the physical manifestation of Logos (or the divine word) and consequently possesses all of the inherent, ineffable perfections which religion and philosophy attribute to the Supreme Being. Three distinct and infinite minds or substances, three co-equal and eternal beings, compose a single Divine Essence (ousia).

This doctrine was formulated in the 4th century CE during the extraordinary Trinitarian or Arian controversy. The several distinct branches of Arianism which sometimes conflicted with each other as well as with the pro-Nicene homoousian creed can be roughly broken down into the following classification:

• Homoiousianism which maintained that the Son was "like in substance" but not necessarily to be identified with the essence of the Father.

• Homoianism which declared that the Son was similar to God the father, without reference to substance or essence. Some supporters of Homoian formulae also supported one of the other descriptions. Other Homoians declared that God the father was so incomparable and ineffably transcendent that even the ideas of likeness, similarity or identity in substance or essence with the subordinate Son and the Holy Spirit were heretical and not justified by the Gospels. They held that the Father was like the Son in some sense but that even to speak of ousia was impertinent speculation.

• Heterousianism (including anomoeanism) which held that God the father and the son were unlike in substance and/or attributes.

All of these positions and the almost innumerable variations on them which developed in the 4th century AD were strongly and tenaciously opposed by Athanasius and other pro-Nicenes who insisted on the doctrine of the homoousian (or as it is called in modern terms consubstantiality), eventually prevailing in the struggle to define Catholic Church dogma for the next two millennia when its use was confirmed by the First Council of Constantinople in 381 or 383.

• Gibbon, Edward. The Decline and Fall of the Roman Empire. Harcourt,Brace and Co. 1960.
• Steenburg, M.C.. A World Full of Arians: A Study of the Arian Debate and the Trinitarian Controversy from ACE 360-380.
• Catholic Encyclopedia: Homoousian