The essence of a thing is a full description of it in its particularity. It is complete information, exhaustive dossier on it. Now information is first and foremost in the mind. For the cup on my desk, for example, the cup’s essence existed in its crafter’s mind before he fashioned it out of matter. He imprinted matter with form with the help of his labor, causing the form to come to reside in “solid reality.”
The cause of essence then is the mind of an intellectual creature. But the essence itself, as an idea or an ideal entity, content of a thought, is fundamentally helpless: it’s causally inefficacious. It cannot on its own accord jump out of my mind and shape matter according to itself.
An uninstantiated essence can be said to be in potentiality; a real object is essence brought into the act of existing.
Thus, essence + existence = concrete object, a suppositum. The suppositum can itself certainly be in numerous acts by “doing various things” proper to it: it can be getting hotter, or expand, or be building a bridge, or be enjoying a cut of coffee. It can initiate and maintain its acts. But the suppositum itself is its essence in act.
Concrete objects exhibit a measure of stability. Once an essence is united with existence, the resulting thing is more-or-less permanent. It resists being destroyed. A baseball can hit a window but fail to break it if its speed is low enough. Now the reason why glass is sturdy is in virtue of the bonds or forces between its molecules and atoms and so forth. However, I am talking about union not between molecules here but rather between the essence of glass and its existence. The forces are of no account; for example, an electron is an elementary particle and cannot be destroyed by being taken apart. But the question of what unites the essence of a particular concrete electron and the electron’s existence remains vital.
We may even suppose that concrete objects have existed forever and were never brought into existence. Assume that no mind equipped with creative power reduced the ideal entities it was contemplating to actual existence. Our argument does not depend on any such event actually occurring. Regardless, a suppositum, being a real thing, can cause and act (be warming up, etc.). But an essence, being an ideal thing, cannot. But an act is something that must be continuously performed by a thing. If an essence cannot perform the act of its own existing, then something else, some real suppositum, must.
Call that thing G1. If it’s real, then it itself is a union of essence and existence. G1‘s essence must in turn be activated by some G2. The problem is merely pushed back. To avoid infinite regress, we need to postulate some G whose essence and existence are not at all distinct but in fact are numerically identical to each other, self-same. G is such that its essence is existence. It is by its very nature in act; moreover, it is perfectly strong, indestructible; its essence, being identical with existence, cannot be cleaved from this existence, and therefore G cannot corrupt or die. It is a thought thinking itself into reality.
That G is what we call God.