Literally, homoousion means "same substance" from homos (same) and ousia (essence) (The Free Dictionary). But it is often translated as “one substance” (e.g., Merriam-Webster).

The word “same” has more than one possible meaning. An article on "Identity" in the Stanford Encyclopedia of Philosophy, states that a distinction is customarily drawn between qualitative and numerical sameness:

Things are qualitatively the same if they share properties. Therefore, things can have different levels of qualitative sameness. The article provides the following examples:

  • Poodles and Great Danes are qualitatively the same because they share the property of being a dog, and such properties as go along with that.
  • But two poodles will have greater qualitative sameness.

On the other hand, things are numerical the same if they have absolute, or total, qualitative sameness. In that case, the two things actually are one thing; therefore the term “numerical.”

So, two perfectly manufactured identical rubber balls on a production line are homoousion but still are two because they do not have total or complete qualitative sameness. They differ, for example, in terms of space occupied.

To apply this to the word "homoousion" in the Nicene Creed:

  • “One substance” implies that the Father and Son share one single (numerically the same) substance while
  • “Same substance” can mean both qualitatively or numerically the same substance. The question is, what did the authors of the creed mean?

Tertullian also described the Father and the Son as of the same substance. He wrote: "For the Father is the entire substance, but the Son is a derivation and portion of the whole" (Against Praxeas, Chapter 9).

In Tertullian, the substance of the Father and the Son are qualitatively the same. They are not numerically the same because the substance of the Son is only a portion of the substance of the Father.

I also know that during the years of intense controversy after the Nicene Creed, many concepts were developed that did not exist in the year 325. So, the question is, how would we know whether the Nicene Creed uses homoousion in the sense of numerical or qualitative sameness?

  • homoousion never appears in scripture and cannot, therefore, be regarded as definitive in the matter of the person of Deity. Homou (meaning 'together') appears one single time.
    – Nigel J
    Commented Dec 16, 2021 at 15:21

2 Answers 2


The authors of the creed definitely meant that God and Christ are one substance. Remember that the creed was formulated against Arius, who claimed that Christ was of a similar substance with the Father, not the same substance. The Christological issue here is that Arius makes Jesus out to be not eternal, whereas God is eternal. Both of these ideas are in direct contradiction with scripture, which says of Christ "In the beginning was the Word, and the Word was with God, and the Word was God... and the Word became flesh, and dwelt among us" and in which Christ says of Himself "I and the Father are one" and "truly I tell you, before Abraham was, I AM [YHWH]." (Emphases mine).

In keeping with the plain truth of scripture, the Nicene fathers are affirming that the Word, which is Jesus Christ, is of one substance with the Father, that He and the Father "are one," and that He is, truly, God. Nothing other than total qualitative sameness is possible here.

Edit: A caveat, the contention is that the Son of God is not eternal, whereas orthodox Christianity holds that He is, in keeping with John 1. The human nature of Christ is created and exists in time. But the Son, consubstantial with the Father, is eternal and is fully and totally God, necessarily, or else John 1 (and Christ's own words, "I and the Father are one) is mistaken.

  • This is quite clearly what was meant by the Nicene fathers. Up-voted +1.
    – Nigel J
    Commented Dec 16, 2021 at 15:24
  • Hi @jaredad7. As explained in the question, "similar substance," "same substance" and "one substance" are three different concepts. "Similar" points to qualitative sameness. "One" is numerical sameness while, as explained in my question, "same" can mean both. So, even if Arius claimed that the Son was of a similar substance, and if the creed intended to oppose that view, the "same" in the creed can still be either qualitative or numerical sameness.
    – Andries
    Commented Dec 17, 2021 at 4:07
  • @Jaredad7 The remainder of your answer appeals to "the plain truth of scripture" which "the Nicene fathers are affirming." With "the plain truth" you mean the orthodox Trinitarian doctrine as the Cappadocians and others developed it much later in the fourth and fifth centuries. To jump right over the first 300 years, when Christ was viewed as subordinate to the Father, and right over the 50 years after Nicaea, when homoousion was rejected, and to interpret the Nicene Creed in terms of a doctrine that developed much later is an anachronism. We have to read the creed in the context of its time.
    – Andries
    Commented Dec 17, 2021 at 4:29

One of the main arguments of classical Arianism was that, since the Father is unbegotten, and the Son begotten (of the Father), then the two must therefore of necessity be of a different nature, inasmuch as one and the same substance cannot be said to be both begotten and unbegotten at the same time, inasmuch as that would lead to a blatant self-contradiction.

  • As such, IF one includes the personal or hypostatic attributes (fatherhood, son-ship, and procession) within the term substance, one must of necessity use homo-ousios, rather than mono-ousios, per your own (correct) understanding of the term's historical meaning, prior to the rise of Arianism.

  • However, IF one excludes personal or hypostatic attributes from the term's definition, than one could use either. This exclusion is ultimately the work of the Cappadocian fathers. It was not present at Nicaea, hence its use of homo- rather than mono-ousios; and, while the Cappadocians themselves have never, to my knowledge, actually changed Greek Nicene terminology, they nevertheless made it possible to translate or render homo-ousios as of one and/or the same substance, in stead of strictly of the same substance, as it has previously been the case.

  • In my view, this answer is balanced and technically correct. Your distinction between homo-ousios and mono-ousios is useful. My only complaint is that you perhaps could have stated it simpler. I think what you say (correct me if I am wrong) is that if we ignore the distinction that the Son was begotten and the Father unbegotten, then we can talk about mono-ousios (one substance) or homo-ousios (same substance), and that the Cappadocians developed the justification for doing so.
    – Andries
    Commented Dec 17, 2021 at 9:03
  • The Wikipedia page on the Cappadocian fathers you refer to is also very informative. For one thing, while, according to @Jaredad7, Arius claimed that Christ was of a “similar substance,” this page distinguishes between “similar substance” and the “outright Arians who taught that the Son was not like the Father” (hetero-ousion). But the more important principle is the “major contributions to the definition of the Trinity” which the Cappadocians made. Since they were all born after the Nicene Creed of 325 was formulated, we must not read that creed as interpreted by the Cappadocians.
    – Andries
    Commented Dec 17, 2021 at 9:06

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