“Homoi-ousias”, which means “like essence” was the Greek word favored by the conservative majority of bishops during the Arian controversy of the fourth century to describe the essential relationship between the Son and the Father. It was put forward as a suggested alternative to the word employed by the Council of Nicea “Homo-ousias”, which means “same essence”, and to the Arian term “Heteroousias”, meaning “different essence”. As Hilary of Poitiers explains in De Synodis (see Hilary of Poitiers on Correct and Incorrect Understandings of Co-essentiality), both ‘Homoiousias’ and ‘Homoousias’, when understood in an orthodox fashion, mean the same thing. If the Son and Father have the same divine nature, or essence, as scripture teaches, then certainly “homoousias” is a fitting word; yet likewise, saying that the Son is like the Father in His essence, meaning, that He is exactly like the Father in His essence, or identical to Him, as can be indicated by “homoiousias”, means the same thing.
But both of these words (as nearly all words do) have a variety of possible meanings; they can each be taken in multiple different ways. For this reason, they were not always meant or understood in an orthodox fashion in the Nicene controversy; both words had ways they could be understood that are heretical. ‘Homoiousias’ allowed for moderate Arians to accept the term because ultimately saying that the Son is of ‘like essence’ with the Father can be taken either as ‘exactly alike’ (which is orthodox), or merely ‘similar, with minor differences’ (which is Arian). For this reason the pro-Nicene, and thus pro-‘homoousian’ minority frequently leveled the charge against the homoiousians that they were semi-arian (even while many of them, ultimately, were not).
Likewise “homoousias” could also be taken in a heretical way, in a modalistic fashion, in which “same essence” was not intended to mean that the Father and Son were distinct persons who shared a common divine nature, but rather that the Father and Son were somehow one subsistent or personal thing.
“Essence” or Greek ‘ousia’ in general was not spoken of nearly as much in the pre-nicene era; it was once the Nicene Council introduced ‘homoousias’ into the Creed that the alternative ‘homoiousias’ became popular. Why? Because not only was it possible to misunderstand ‘homoousias’ in such a way that it would mean that the Father and Son were ultimately a single person, but the word actually already had a history of being used that way by the time of the Arian controversy. Thus, many orthodox bishops desired another term to use.
“Homoousias” was associated with Sabellius, an early modalist, and was also used by later ante-nicene modalist Paul of Samosata. The local council which condemned his teaching as heretical actually condemned the word “homoousias” as heretical, as well, on the basis of its modalistic usage. For this reason when this word which had a strong association with modalism, and tendency to be understood in a modalistic way, was employed by the Nicene Council, many of the church fathers at the time objected, although the orthodox ‘Homoousian’ fathers made efforts to explain to orthodox meaning of the word which they intended to communicate by it.
Eventually, with much explaining, “homoousias”, despite the grave concern by many that the word was modalistic, won the day, eventually being accepted at the Council to Constantinople in 381. “Homoiousias” came to be associated with the “semi-arians”, and eventually with Arianism at large, as time went on, in large part thanks to the polemics of semi-modalists in centuries following. From the time of the Nicene controversy onward, it has been a popular polemic against anyone not favorable term ‘homoousias’ to label them as being in some way Arian, even when the difference is merely one of terminology and not meaning.
However, this language of the Son being “homoousias” with the Father did not take long to again take on an ultimately modalistic meaning, as semi-modalism redefined the entire concept of consubstantiality which the word stood for to mean that the Father and Son were ultimately a single person, “God the Trinity”. Such redefining can be seen in the Fourth Lateran Council, as well as in the influential writings of Augustine (see Augustine’s Trinitarian Heresy). The concept of co-essentiality was twisted to no longer mean that the persons of the Son and Holy Spirit have the same divine nature as the Father, but rather to say that the Father, Son, and Spirit are all one subsistent thing, or person. Thus a term that had indicated generic unity, or identicality of nature, was now altered to indicate that the three persons of the Trinity were numerically one, or one person.
Those homoiousian Christians of the fourth century then, as well as those who favored the term “homoian” (which sought to leave the unscriptural term “essence” or “ousia” out of the discussion altogether, and merely confess that the Son is “like” the Father) were ultimately vindicated in their misgivings about the term “homoousias”. They protested it for fear it was Sabellian- that was its history, and it was worried that it would again be taken in such a way in the future. The Homoiousians and Homoians (who were slandered as being Arian by the Homoousian minority) were right; this is exactly what happened.
Although they are often slandered for their misgivings about the word, the Homoian and Homoiousian bishops of the fourth century have ultimately been vindicated in respect to their distrust of the word ‘homoousias’. The very thing they warned could happen did, in the post-nicene era.
While homoousian consubstantiality, as intended by its original authors such as Athanasius, is entirely orthodox, it introduced a shift in emphasis from the persons of the Trinity to the divine nature They share, and an emphasis on this one divine nature being the “one God” of Christianity. Perhaps in overreaction to Arianism, Homoousian Christians eventually gave up the confession that the one God is the Father, and instead emphasized the divine nature as Christianity’s one God.
This shift in language was doomed to result in semi-modalism. In scripture, the “one God” is always a person, and such is the natural way to think of God: as personal. Scripture, however, as the early church did, specifies that this one God is the person of the Father in particular; the Son is His Son, the Holy Spirit, His Spirit (see I believe in one God, the Father Almighty). By shifting the focus onto the essence as the one God of Christianity, Homoousian Christians in the post-nicene era doomed the church to fall into thinking of the essence as a person, therefore, since the one God is a person. Using what was ultimately the title of a single person for the divine nature shared by all three persons led to natural confusion, and what we see down to the present day, a personifying of the divine nature as a fourth person in the Trinity (see Semi-modalism and the Introduction of a Four-Person Trinity).
The homoousians didn’t merely pioneer this change in language, but emphasized conceptually that monotheism depended on the fact that there is one divine nature shared by the persons of the Trinity. While this fact is true, the unity of God does not depend on the fact that there is one divine nature, but on the fact that there is one Father, one supreme uncaused Cause of all, and Supreme Authority over all. For in the case of three men there is also a unity of nature, one human nature being common to all human persons; yet all human persons are not one man, but many men. And besides, even the fact that the persons all share one divine nature is dependent on the person of the Father, since He is in Himself the very definition of that divine nature, without cause or source; and yet is Himself the Source of that divine nature to His Son and Spirit, as They have the divine nature from the Father in eternal generation and procession, respectively.
This emphasis, then, on the divine nature as the unity of God, instead of the Father, has proven detrimental throughout the many centuries since. Semi-modalism easily grows out of such an emphasis, because, as mentioned above, three persons merely being of one nature does not make them “one God”, any more than three men being of one human nature makes them one man. If then, this unity of nature is insisted on as the explanation of Christian monotheism, is necessarily must be altered to mean something beyond a mere unity of nature: a unity of person. To deny the charge of tritheism on the basis of a Nicene understanding of co-essentiality alone is impossible; therefore, since the classical grounding of monotheism was abandoned, the new one developed was to redefine co-essentiality to mean not merely that the three persons share one essence, but are one “being”; a vague term, which, in fact, ends up being conceptually equated with person (see also Equivocation Over the Term “Person”).
Because this is recognized as modalistic to treat the three persons as one person, the language of the three being one “person” was never embraced by the church broadly; yet conceptually, that is what co-essentiality has been redesigned to signify in the post-nicene understanding. Accordingly, the response of those committed to a post-nicene scholastic redefinition of co-essentiality, as can be seen in the Fourth Lateran Council (see The Grievous Error of the Fourth Lateran Council), is to accuse those articulating a classical understanding of co-essentiality of being tritheists, failing to recognize that the grounding of Christian monotheism is not that the Son and Spirit of God share His divine nature (although this is true), but that there is one supreme uncaused Cause of all, Who is one Supreme Authority over all, the Father (see Why There is Only One God: One Supreme Cause and Why There is Only One God: Headship).
It was not, therefore, the emphasis on the persons of the Trinity sharing one essence, or one divine nature, that was the fatal flaw of homoousian theology, so to speak, but the Homoousians’ emphasis of this unity of nature as the grounding of Christian monotheism, combined with the abandonment of the classical grounding of Christian monotheism. This unbiblical shift led directly into the widespread acceptance of semi-modalism, to the destruction of the classical trinitarianism the original Homoousians contended for.
Arianism, with its emphasis on the Father’s role as the one God, the supreme uncaused Cause of All, and the Supreme Authority over all, served as a catalyst for this change, as these ideas naturally became associated with a heretical Christology. The result of this was important aspects of classical trinitarianism being divided up between Arianism and the Homoousians; the Arians emphasizing the Father as the one God, and ground of monotheism, and the Homoousians emphasizing the co-divinity of the persons of the Son and Holy Spirit with the Father. While Arians always rejected the Homoousian emphasis, initially Homoousian Christians accepted the Arian emphasis as an aspect of orthodox trinitarianism. But as time went on, Arian association with these ideas led to a de-emphasizing of these concepts in Homoousian theology, although they were never actually repudiated. Arianism can thus be argued to have done more damage to the cause of classical trinitarianism by stigmatizing elements of classical trinitarianism by association with its heresy than it did by actually promulgating a heretical Christology, which over the scheme of history has ultimately not been successful in maintaining a large following. But by attacking the classical trinitarian doctrine of the Son and Spirit’s co-divinity with the Father, Arianism enticed the church to over-react in the opposite direction by overemphasizing the doctrine of co-essentiality to the eclipsing of other elements of classical trinitarianism.
The first cracks in Homoousian theology can be seen within its first generation, which accepted the classical trinitarian doctrines that the Father is the one God, the supreme uncaused Cause of all, and the Supreme Authority over all, as they shifted emphasis from these doctrines to the fact that the Son and Spirit share the Father’s divine nature. In order to emphasize the truth of the Son and Spirit’s co-essentiality with the Father, otherwise orthodox Homoousian theologians began twisting scripture to read it as speaking of the divine nature, rather than the person of the Father, in certain passages; the first intimations of the semi-modalism that would sweep the church in the following generations.
For example, Athanasius wrote:
“For what is nearer [God] than the Cherubim or the Seraphim? And yet they, not even seeing Him, nor standing on their feet, nor even with bare, but as it were with veiled faces, offer their praises, with untiring lips doing nought else but glorify the divine and ineffable nature with the Trisagion. And nowhere has any one of the divinely speaking prophets, men specially selected for such vision, reported to us that in the first utterance of the word Holy the voice is raised aloud, while in the second it is lower, but in the third, quite low,—and that consequently the first utterance denotes lordship, the second subordination, and the third marks a yet lower degree. But away with the folly of these haters of God and senseless men. For the Triad, praised, reverenced, and adored, is one and indivisible and without degrees (ἀσχηματιστός). It is united without confusion, just as the Monad also is distinguished without separation. For the fact of those venerable living creatures (Isa. vi.; Rev. iv. 8) offering their praises three times, saying ‘Holy, Holy, Holy,’ proves that the Three Subsistences443 are perfect, just as in saying ‘Lord,’ they declare the One Essence.” (Athanasius, On Luke 10:22)
Ambrose of Milan, of the first generation of post-nicene Homoousians, similarly wrote:
“Dominations and powers fall down before Him — you speak evil of His Name! All His Saints adore Him, but the Son of God adores not, nor the Holy Spirit. The seraphim say: Holy, Holy, Holy! Isaiah 6:3
107. What means this threefold utterance of the same name Holy? If thrice repeated, why is it but one act of praise? If one act of praise, why a threefold repetition? Why the threefold repetition, unless that the Father and the Son and the Holy Spirit are one in holiness? The seraph spoke the name, not once, lest he should exclude the Son; not twice, lest he should pass by the Holy Spirit; not four times, lest he should conjoin created beings [in the praise of the Creator]. Furthermore, to show that the Godhead of the Trinity is One, he, after the threefold Holy, added in the singular number the Lord God of Sabaoth. Holy, therefore, is the Father, holy the Son, holy likewise the Spirit of God, and therefore is the Trinity adored, but adores not, and is praised, but praises not.” (Ambrose of Milan, De Fide, Book 2, Chapter 12)
Both Athanasius and Ambrose explain the vision of Isaiah 6 as pertaining to the whole Trinity, instead of the Father, as can be understood from the parallel passage in Revelation 4 (see Examining Scripture: The “Lord God Almighty” of Revelation Chapter 4). They both read a Homoousian understanding of the Trinity -with its supreme emphasis on the unity of the divine nature- into the passage, explaining the three repetitions of “Holy” as indicating the three persons, and the singular “Lord God” as indicating the singular essence, or divine nature. This interpretation is seriously flawed, choosing to forcibly insert Homoousian theology into scripture where it is not spoken of, contrary to the interpretation offered in the New Testament in Revelation 4 which clarifies this as referring to the person of the Father, “the Lord God Almighty,” alone.
What may also be noted here is that although both Ambrose and Athanasius usually avoid treating the Trinity as a person (unlike later generations of Homoousian theologians), by making this strained interpretation of the passage in order to seemingly provide more biblical support for Nicene trinitarianism, they fall into regarding the Trinity as a single person; for the vision in Isaiah 6 clearly treats the “Lord God” on the throne not as an impersonal essence, as the divine nature considered in abstract is, but as a person, who speaks to Isaiah and sends him as a prophet.
By taking passages of scripture that refer to a single person of the Trinity and saying they speak of the essence, the groundwork for future semi-modalism was laid, which would blatantly treat the essence or Trinity as a whole as a person. Although this misinterpretation can be regarded as a relatively minor mistake on its own, it would be amplified into a completely different theology by later theologians, such as Augustine of Hippo (see Augustine vs. Athanasius on the Identity of the “One God”).