Homoousios

Homoousios — "of one essence" — is the word the Church set in the Nicene Creed to confess that the Son is fully and truly God, sharing the very being of the Father, not a lesser or created thing.

For Inquirers

The entire Christian faith turns on one question about Jesus, and the answer is not obvious: is He really God — God as much as the Father is — or is He the greatest and highest thing God ever made, but still, in the end, a creature? For a while the Church nearly split over exactly this, because a great many people wanted to say Jesus was God-like, wonderful beyond all others, without saying He simply is God.

The word the Church finally chose to settle it is homoousios, an old Greek word that means "of the same essence" — made, so to speak, of the very same stuff. To say the Son is homoousios with the Father is to say that whatever the Father is, the Son is that too, completely: not a copy, not a junior god, not the finest of God's works, but truly God from truly God. The old creed reaches for light to picture it — the brightness that pours out from a flame is not a weaker, second-hand light; it is the same light, streaming out. So the Son streams eternally from the Father, and is everything the Father is.

Here is the part that still amazes: the rival word, the one that meant merely similar rather than same, differed from it by a single letter. People really did feel the whole faith hanging on that one letter — and they were right to. Because if Jesus were only God-like, then when He came down to rescue us it would be a creature reaching for us, not God Himself reaching all the way down to where we are. The word is a fence built around the most important thing there is to say: that in Jesus, God did not send a substitute. He came Himself.

Overview

Homoousios (Greek ὁμοούσιος, from homós, "same," + ousía, "essence") means "of one essence" or "consubstantial." It names what the Son is: He shares the one, undivided divine being of the Father.
It was placed in the Nicene Creed at the First Ecumenical Council (325) to confess the Son as "of one substance with the Father," ruling out Arianism, which held the Son to be a creature.
The word is not found in Scripture; the Council judged it the one term the Son's opponents could not empty of meaning.
Its semi-Arian rival was homoioúsios, "of like essence" — the two differ by a single Greek letter (iota), the origin of "not one iota of difference." Later the same confession secured the full divinity of the Holy Spirit.

Summary

Arius taught that the Son is a lesser, made being — the first and highest of creatures, but a creature still — and he taught it with a slogan the Council would quote back in order to condemn it: of the Son, "there was a time when he was not." The First Ecumenical Council at Nicaea (325) answered by fixing the Son's status in a single load-bearing phrase of the Creed: "very God of very God, begotten, not made, being of one substance (ὁμοούσιον, consubstantialem) with the Father." Whoever taught otherwise, the Council added, stood condemned by name: any who say "there was a time when the Son of God was not… or that he is of a different substance or essence from the Father, or that he is a creature."[^nicaea]

The Council chose a word the Bible never uses, and did so deliberately, because the Arians could survive anything gentler. They were happy to grant the Son every scriptural title — Son, Word, Image, Wisdom, even "God" — and then quietly drain each one, treating them as honours bestowed on an exalted creature. Every biblical phrase they could bend; what they could not bend was a plain metaphysical word that said the Son is not merely called divine but is what the Father is. St. Athanasius spent his life defending exactly that word, because to give it up was to hand the faith back to those who would keep the words of Scripture while losing its God.

The most dangerous opposition was never open denial but a near-miss. A large party — the semi-Arians — proposed homoioúsios, "of like essence," differing from homooúsios by a single letter, the iota. Athanasius, recounting the councils of his day, notes that these men "accepted the whole proceedings of the Council, except that they complained of the word 'Coessential,' as obscure and so open to suspicion."[^synods] "Like" is fatally weaker than "same": two things can be perfectly alike and remain two distinct beings, the way a portrait is like its subject and is not the man. To say the Son is only like God still leaves Him outside God, a second and lesser being however exalted; "of the same essence" alone says what the Church actually confesses — the one God's own being, eternally begotten and never made.

Salvation itself rides on getting this right. Athanasius pressed the point relentlessly: the whole meaning of Christmas and Easter is that God Himself came down to lift us up. "He was made man," he writes, "that we might be made God."[^athdeif] If the Son were a creature, then a creature came down, a creature died and rose, and we are joined at the end to a creature, not to God at all. It is precisely because the Son is homoousios with the Father that His becoming man reaches all the way to God, and can carry us there.

What Nicaea confessed of the Son, the Second Ecumenical Council (381) confessed of the Holy Spirit — worshipped and glorified together with the Father and the Son — so that the single word, "of one essence," came to guard the full divinity of all three Persons, and with it the whole doctrine of the Trinity.

See also Ousia, Nicaea I, Arianism, St. Athanasius, and The Incarnation.

Sources

[^nicaea]: First Council of Nicaea (A.D. 325), the Nicene Creed — "the Son of God… of the substance of the Father, God of God, Light of Light, very God of very God, begotten, not made, being of one substance (ὁμοούσιον, consubstantialem) with the Father," and the anathema against those who say "there was a time when the Son of God was not… or that he is of a different substance or essence… or that he is a creature.". Nicaea I [^synods]: St. Athanasius, De Synodis (On the Councils of Ariminum and Seleucia) — the semi-Arian majority "accepted the whole proceedings of the Council, except that they complained of the word 'Coessential,' as obscure and so open to suspicion." Athanasius' great defense of the Nicene homoousios against the homoiousios ("like in essence") party. [^athdeif]: St. Athanasius, On the Incarnation of the Word, §54 — "For He was made man that we might be made God; and He manifested Himself by a body that we might receive the idea of the unseen Father." The soteriological stake of the Son's true divinity. [^owiota]: OrthodoxWiki, homoousios / Semi-Arianism — the single-iota difference between homoousios and homoiousios and the phrase "not one iota of difference.".