Homoiousios ( from , hómoios, "similar" and , ousía, "essence, being") is a Christian theological term, coined in the 4th century to identify a distinct group of Christian theologians who held the belief that God the Son was of a similar, but not identical, essence (or substance) with God the Father.
Overview
It is often claimed that Homoiousianism arose as an attempt to reconcile two opposing teachings, namely, Homoousianism and Homoianism:
Homoousios is a keyword in the Nicene Creed of the year 325 and means "same substance." Homoousianism was a continuation of that concept and taught that the Son is of the same (ὁμός, homós, "same") substance as the Father. Consequently, the Son is co-equal and co-eternal with the Father. Homoianism, on the other hand, refused to use the term οὐσία (ousía, "essence"). Homoians believed that the Son is "like" or "similar" (ὅμοιος, hómoios) to the Father but subordinate to the Father.
It is then proposed that Homoiousian (similar substance) theology was an attempt to reconcile the Homoousian (same substance) theology with the Homoian notion of similarity.
However, Homoiousianism was “most prominently associated with … Basil of Ancyra” and "the term homoiousios plays no role in Basil's surviving texts," implying that such a compromise was not the purpose. More recently, Lewis Ayres proposed that Homoiousianism was not merely a compromise but "a significant and persistent strand in earlier eastern theology."
There are indications that this theology was a restatement or development of the theology of Eusebius of Caesarea, as stated in the letter he wrote to his home church after the Nicene Council, to explain why he accepted that Creed:
Ritter described Homoiousianism "as the right wing of the Eusebian party.” “Basil … prefers the term 'image of the ousia' to define the Son's relationship to the Father; it is worth noting that this term was favoured by Eusebius of Caesarea … and also is found in the Second ('Dedication') Creed of Antloch 341.”
Eusebius was “universally acknowledged to be the most scholarly bishop of his day.”
A response to Neo-Arianism
Both Lewis Ayres and R.P.C. Hanson stated that the formulation of Homoiousian theology in 358 by a council of bishops called by Basil of Ancyra was a response to, what Ayres calls, "the emerging shape of Heterousian theology" in the form of the creed of “Sirmium 357,” which was based on the teachings of Aetius. Hanson refers to this as "Neo-Arianism" and as “a new and radical theology” that appears for the first time in the extant ancient records in the form of the “Second Creed of Sirmium of 357,” afterward approved by a larger synod at Antioch (probably in AD 358). 'Neo-Arianism' may be an appropriate name because it was “a development" of Arius' theology.
“The statement which emerged from this council … marks the emergence of a new and coherent theological point of view. This is the theology of those whom Epiphanius, quite undeservedly, calls 'Semi-Arians', but who are usually today thought of as Homoiousians, a designation which is more accurate.”
Doctrine
“The statement which emerged from this council … marks the emergence of a new and coherent theological point of view. This is the theology of those whom Epiphanius, quite undeservedly, calls 'Semi-Arians', but who are usually today thought of as Homoiousians, a designation which is more accurate.”
This statement was written by Basil of Ancyra himself and “is of the highest importance for an understanding of Homoiousian theology.” It includes “nineteen anathemas which reveal more clearly the position which Basil is attacking."
Against Homoianism
Homoianism was a dominant Christology during the mid-fourth century. It refused to use ousia (substance) language in the formulation of any statement of faith. Against them, Basil insisted that substance language is necessary to reflect the closeness of the Father and Son expressed by the concepts “Father/Son” and “begotten.” He wrote:
“God must be both Father and creator” (of His Son). “If we remove this resemblance of ousia,” the Son is merely a created being; “not a Son.” Since human sons are like their fathers, the Son of God is like His Father. “The salient irreducible element” in a father/son relationship is “the begetting of a living being that is like in ousia.” “If the Father gives the Son to have life in himself (John 5:26) … then the Son must have the same life and thus have ‘everything according to essence and absolutely as does the Father’.” (Lewis Ayres, page152)