When in A.D. 325 the bishops of the Council of Nicaea declared that Jesus Christ is begotten “from the substance of the Father” and “of one substance” with him, they probably did not foresee the momentous dogmatic consequences of their decision. They clearly did not believe they were implementing a theological revolution. Their choice of homoousios, though controversial, was intended primarily to exclude Arius’s ontological subordination of Christ, a subordination expressed in his slogan “There was once when the Son was not.” Such a clear and unambiguous assertion of the creaturehood of the Son was just too much. The Nicene bishops probably disagreed on the precise meaning of the term homoousios. They simply knew that it was a term to which Arius could not subscribe. The strategy worked. Arius and his supporters were exiled, the bishops returned to their episcopal sees, and the homoousion was promptly forgotten. The Nicene confession did not become the official creed of the imperial Church. The homoousion did not become the dogmatic criterion by which to interpret the identity of Christ. Bishops continued to teach as they had done before the council.
Yet the Council of Nicaea was not completely forgotten. Approximately twenty-five to thirty years later, St Athanasius of Alexandria would recover the Nicene confession and reassert the defining significance of the homoousion. The crucial document is Athanasius’s treatise De Decretis. Athanasius here interprets the Nicene Fathers as intentionally asserting the divine nature of Christ Jesus and excluding any and all suggestions of creaturehood. With reference to the Nicene claim that Christ is “from the substance of the Father,” Athanasius writes:
The council wished to banish the impious phrases of the Arians and to inscribe the words confessed by the Scriptures: that the Son is not from non-being but from God; that he is Word and Wisdom, neither creature nor something made, but from the Father as his own (idion) offspring. But the party of Eusebius, compelled by their longstanding perversity, wished the designation of his being “from God” to be taken as something in common with us and the Word of God to be no different from us in this respect, as it is written: “one God from whom are all things” (1 Cor 8:6) and “the old things have passed away; behold all that is new has come to be; and all this is from God” (2 Cor 5:17, 18). So the fathers of the council, seeing their deceit and the machinations of their impiety, finally found it necessary to proclaim the “from God” more clearly and to write “the Son is from the essence of the Father” (ek tēs ousias tou theou), so that “from God” may not be considered to be the same and equal in the case of the Son as it is with things that have come to be; but that it may be confessed that while all others are creatures, the Word is uniquely from the Father. For even if all things are said to be from God, this is altogether otherwise than how the Son is. In the case of created things, they are said to be from God in that they do not exist randomly and unaccountably; neither do they attain their origination by chance, as those who speak of an origination that comes about from the intertwining of atoms and of like parts; nor, as certain heretics say, is there another creator, nor, as again others say, do all things have their subsistence through some angels. Rather, all things are said to be from God because the existent God, by himself and through the Word, brought all things that formerly did not exist into being. But the Word is said to be and is alone from the Father because he is not a creature; and the Son’s being “from the essence of the Father” is indicative of this sense, which does not pertain to anything that has come into being. (Decr. 19)
The Son is thus declared to be “from the substance of the Father” in order to clearly differentiate him from all beings who have been made by the Father from out of nothing. Christ is not a creature. He is God from God, light from light, true God from true God.
With reference to the Nicene claim that the Son is “homoousios with the Father,” Athanasius explains that the Nicene Fathers wanted to assert not just the likeness of the Son to the Father but their unity in being:
But the bishops … found it necessary again to gather together the sense of the Scriptures and to speak more clearly the things which they said before, and to write, “the Son is one in essence (homoousion) with the Father,” in order to signify that the Son is not only like, but from the Father as the same in likeness (tauton tē homoiōsei), and in order to show that the likeness and inalterability of the Son is other than the imitative likeness that is ascribed to us and which we attain through virtue by keeping the commandments. … But since the generation of the Son from the Father is other than that which pertains to the nature of human beings and he is not only like (homoios) but also inseparable from the essence (ousia) of the Father and he and the Father are one, as he himself said (Jn 10:30), and the Word is always in the Father and the Father in the Word (cf. Jn 10:38)—as is the radiance in relation to the light (for this is what the phrase means)—the council, understanding all this, aptly wrote “one in essence” (homoousion). They did this in order to overturn the perversity of the hypocrites and to show that the Word is other than the things which come to be. For immediately after writing it, they added: “But those who say that the Son of God is from non-being or is a creature or changeable or made or from another essence (ousia), these the holy and catholic Church anathematizes.” In saying this, they made it manifestly clear that “from the essence” and “of one essence” are abrogations of the trite slogans of the impious: such as that he is a “creature” and “made” and something which has come into being (genēton) and changeable and that he was not before he was generated. The one who thinks such things is contradicting the council. (Decr. 20)
The symbol (paradeigma) of the light and the radiance also has the same meaning. For the sacred writers did not say that the Word was related to God like a fire which is ignited from the heat of the sun and which is usually extinguished again, for this is an external product and creation of its maker. But they all preached of him as Radiance, in order to disclose his being properly and inseparably from the essence and his unity with the Father. Thus will his unchangeability and inalterability also be truly secured, for how can he be unchangeable and inalterable if he is not the proper (idion) offspring of the Father’s essence? For it is necessary, in regard to this also, to safeguard his identity with his own Father. Since this explanation is thus shown to be pious, the enemies of Christ should not be shocked by the “homoousios” either, since this term also has a sound sense and rationale. For if we say that the Word is from the essence of God (let this at last be confessed by them!), what is that except to say that he is truly and eternally of the essence from which he is begotten? For he is not different in kind, as if he were something foreign and dissimilar (anomoion) that is mixed in with the essence of the Father. Nor is his likeness merely extrinsic, as if he were in some other respect or completely of a different essence (heteroousios), just as brass shines like gold and silver and tin. These are foreign to one another and of different natures and are separate in their natures and their powers. Brass is not proper (idion) to gold, any more than a pigeon is from a dove. Even though they are considered to be like (homoia) each other, they are nevertheless different in essence. Therefore, if that is how the Son is, then he is a creature like us and not one in essence (homoousios). But if the Son is Word, Wisdom, Image of the Father, and Radiance, then it follows reasonably that he is “one in essence.” (Decr. 23)
Eusebius of Caesarea would no doubt have dissented from Athanasius’s account; but the Alexandrian’s interpretation of the dogmatic significance of the Nicene homoousion eventually won the day, was confirmed at the 381 Council of Constantinople and ultimately received into the dogmatic consciousness of the Church. From this point on there could be no going-back on the Nicene definition. The jigsaw puzzle was complete. All ontological subordination of the eternal Son is irreversibly excluded: Jesus Christ is of one being/essence/substance with God the Father. Alasdair Heron elaborates on the significance of the Athanasian victory:
First of all—and this is by no means unimportant—it must be stressed that so far as Athanasius was concerned it was not the word homoousios itself that was of central importance, but what the word stood for. There was no substantial change in his position when he came more and more to use homoousios in his writings: it served simply to focus and concentrate the entire debate with Arianism. What it meant for Athanasius was simply this: that the reality of God himself is present with us and for us in Christ. “One ousia” means “one divinity”, “one activity”, “one presence”, “one glory”, “one power and energy”: all that the Father is, the Son is also, except that the Father is Father, the Son, Son. This was the decisive difference between Athanasius and Arianism; for any assertion of this kind was, in the Arian horizon, strictly incorrect and untrue. For Arius and his followers, however, the point might be expressed, decorated or qualified, the Son is not God as the Father is God; for Athanasius, he is. (“Homoousios with the Father,” in The Incarnation, p. 67)
But if the Son (and Spirit) is truly divine, then deity itself must be reenvisioned. Simple unitarian construals of divinity are no longer possible; created intermediaries between the Father and the world are excluded. The One God is the Father with his equally divine Son and equally divine Spirit.