Eunomius had relied on Aristotelian metaphysics to construct a theology where the Son and Spirit were “energies” of the Father. In Eunomius’ thought, this also entailed that they were distinct from the Father’s essence and of a lower quality.
In rejecting this way of thinking, Gregory gives us some insights on his own understanding of the relationship between essence and energies, and in keeping with his commitment to simplicity (of which we will have more to say in a bit), he identifies the energy with the essence. Thus, even if the Son and the Spirit were energies of the Father, they would also be of his essence. Gregory writes:
But it is worth a moment’s while now to consider how energies ‘follow’ [Eunomius’ terminology sw.] beings: what these energies are essentially: whether different to the beings which they ‘follow,’ or part of them, and of their inmost nature: and then, if different, how and whence they arise: if the same, how they have got cut off from them, and instead of co-existing ‘follow’ them externally only. This is necessary, for we cannot learn all at once from his words, whether some natural necessity compels the ‘energy,’ whatever that may be, to ‘follow’ the being, in the way heat and vapor follow fire, and the various exhalations the bodies which produce them. Still I do not think that he would affirm that we should consider the being of God to be something heterogeneous and composite, having the energy inalienably contained in the idea of itself, like an ‘accident’ in some subject-matter: he must mean that the beings, deliberately and voluntarily moved, produce by themselves the desired result. But, if this be so, who would style this free result of intention as one of its external consequences? We have never heard of such an expression used in common parlance in such cases; the energy of the worker of anything is not said to ‘follow’ that worker. We cannot separate one from the other and leave one behind by itself: but, when one mentions the energy, one comprehends in the idea that which is moved with the energy, and when one mentions the worker one implies at once the unmentioned energy.
An illustration will make our meaning clearer. We say a man works in iron, or in wood, or in anything else. This single expression conveys at once the idea of the working and of the artificer, so that if we withdraw the one, the other has no existence. If then they are thus thought of together, i.e. the energy and he who exercises it, how in this case can there be said to “follow” upon the first being the energy which produces the second being, like a sort of go-between to both, and neither coalescing with the nature of the first, nor combining with the second: separated from the first because it is not its very nature, but only the exercise of its nature, and from that which results afterwards because it does not therein reproduce a mere energy, but an active being.
~Against Eunomius 1.17
Notice that for Gregory the essence and the energy cannot be separated, and the one is comprehended by mention of the other. When he writes that the energy is “not its very nature,” we see that this is a negative hypothetical. It is one option that he is placing in his opponent’s mouth and rejecting because of its incompatibility with the fact that the Son and Spirit are active beings.
Even if the Son and Spirit were energies of the Father, and Gregory does not believe that this is the full case at all, they would still be of the Father’s essence, and thus equal in dignity, worth, honor, etc.