In Summa Contra Gentiles 2.9.4, Aquinas writes
Just as active power is something acting, so is its essence something being [Sicut potentia activa est aliquid agens, ita essentia eius est aliquid ens].
I have two questions about this sentence.
Well, I'm not sure if I understand your question. Thomas is equating essence with entity (ens, entis), not with being (esse).
Entities participate in the actus essendi (= what you call existence, although it is not the same): "Ens enim subsistens est quod habet esse tamquam esse quod est ..." (Cf. Quattuor libros Sententiarum, III Sent., d. 6, q. 2, a.).
Entity (ens) includes, therefore, the essence and the esse; thus, the entity can be characterized as essentia quod habet esse, the essence that possesses esse. This esse, adds Thomas, is that by which the being subsists: "quo aliud subsistit," that is, by which it exists by itself and in itself, or by which it possesses a certain particular mode of existence.
Existence is an effect of esse and not the other way around; in the same maner that essence is an effect of esse. That's why esse = actus essendi =/= existence =/= essence
While @Ian answer is incredibly accurate, I would like to paint a similar yet differing view regarding the matter of esse as an effect of essence.
First, to support his answer, we do see linguistically that esse is literally the prefix for essence, if we divide the word itself into esse- and +nce. This is interesting, as noted, esse is not what Thomas is referring to, rather, he utilizes entity, ens,entis instead. An entity itself is quite the concept, and has had many different views put forth throughout history. An entity can be something purely material, as in a material entity, it can be something purely immaterial as in an immaterial entity, think of things such as "ghosts", or it can be a merger of the two, which it seems as though Thomas is referring to entities as such, mergers of the immaterial and the material.
Things get sort of fuzzy however, when we look at the definitions for being, esse in the modern moment. Which is defined as such:
-Being: Existence, being alive, living; The nature or essence of a person; a real or imaginary living creature or entity, especially an intelligent one.
As we can see, the definition itself is arguing for multiple different views. If being is solely determined via being alive, existing as a living creature, then we see death as the end of being. If being is simply the nature or essence of a person, we see that such things can be preserved through death. This idea is supported via the suggestion that we can communicate with our ancestors, even after they have physically left the planet. If being is solely regarding a real or imaginary creature or entity, even one of intelligence, then we see that the prior two understandings are meaningless, for we would all derive our own meaning in regard to what makes something categorically qualified for being.
Thus, I would like to put forth that essence itself is what precedes both being, esse, and existence. For we would be able to support the definitions of being in both the immaterial and material sense, allowing for entities themselves to be both physical and non-physical or metaphysical. A soul (immaterial) is being a soul, even after death, just as that soul was a soul in a body (material) in physical life. Of course, the very latter notion of being, is perhaps best for the category of fiction, such as novels and other forms of literature/entertainment. In reality, however, we can indeed utilize being, esse, as a term that expresses duality between both the material and immaterial, existence and essence (Essence = immaterial, thus being first, prior to material existence. Existence = material, thus being second, after the acknowledgement of immaterial origin.)
To summarize, the order posited via my argument is as such:
Essence (1) --> Existence (2)
-Being, esse (1 & 2)(Referencing inclusion under both essence and existence.)
-Entity, ens (1 & 2)(Referencing inclusion under both essence and existence.)
Therefore, we see that entities may be just as we may be entities. Entities may be existent physically (materially), just as they may be metaphysically (immaterially). The important thing to note, as mentioned above, is that essence is in reference to the immaterial, whereas existence is in reference to the material. This is precisely why we don't say that an entity exists metaphysically, for its being down to pure essence, is not dependent upon material existence.
