Thomas explicitly asserts that God is numerically identical to His essence. God does not have divinity as His nature: He is His own divinity. As we have seen, God is not an abstract object, a post rem property, or a universal. So, what could it mean to say that God is identical to His essence or nature?
This isn’t as strange a claim as it seems at first glance. Thomas explains at the end of De Ente et Essentia, chapter 1, that the word ‘essence’ (essentia) can be used in two senses. These two senses correspond to referring to the human essence as humanity and referring to it as man. In the first sense an essence is a species of form, considered entirely apart from any individuating matter. This corresponds to a mere universal, a logical being, a species to which individual substances can belong but which is distinct from any particular substance. In the second sense, the term ‘essence’ can refer to both the form and the matter of a particular substance, while referring to the matter only “implicitly and indistinctly.” In this sense, we can say that Socrates is an essence. To keep things clear, let’s use the phrase ‘real essence’ to express the second sense. Real essences are particular individuals, metaphysical parts or components of ordinary substances.
No real essence, not even the real essence of a created thing like a frog or a drop of water, is an abstract object, post rem property, or a universal. All real essences are “concrete” in the modern sense. The real essence of a frog includes that frog’s individual substantial form, which is a real being that does real causal work. The real essence also includes the frog’s prime matter, although it does so only indistinctly, leaving out the facts about the particular spatiotemporal and geometrical properties of the matter. Consequently, the frog’s essence is in some sense an individual frog, but it is a frog stripped of all its features except its bare frogginess.
The ordinary frog is not strictly identical to its real essence, since the frog has determinate spatial features, features that its real essence lacks. However, if there are immaterial substances (like angels), we could suppose that these immaterial substances are strictly identical to their individual real essences. Therefore, to claim that God is identical to His real essence is simply to claim that God is immaterial.
Here is Thomas’s argument for this claim in Question 3, article 4:
[I]n things not composed of matter and form, in which individualization is not due to individual matter—that is to say, to “this” matter—the very forms being individualized of themselves—it is necessary the forms themselves should be subsisting “supposita.” Therefore “suppositum” and nature in them are identified. Since God then is not composed of matter and form, He must be His own Godhead, His own Life, and whatever else is thus predicated of Him.
Thomas uses the word ‘suppositum’ to refer to a particular substance as a particular thing. Material substances are individuated by their matter, and so they cannot be simply identified with their substantial forms (which derive their particular identities and mutual distinctness from the identities of their individual parcels of matter). Since the nature or essence of a substance is entirely determined by its substantial form, this means that material substances are numerically distinct from their natures or essences. Material substances are (by virtue of the matter they contain) intrinsically and per se distinct from one another, but this is not true of their natures. As we saw in chapter 3, the forms and natures of material substances in the same species are not primitively distinct from each other. Their mutual distinctness depends on the mutual distinctness of their parcels of prime matter. An immaterial substance, containing no matter, must be distinct in nature from all other substances. Hence, the particular immaterial substance and its particular essence are one and the same. Since God is immaterial, He must be identical to His own nature.
The Analytic Thomist 