I refer to the following argument as a "metaphysical argument," because it has a close resemblance to both cosmological and ontological arguments. I think it deserves its own category. The argument I'm referring to is the proof found in St. Thomas Aquinas's small tract, De Ente et Essentia (On Being and Essence).
Now, I've written quite a bit about the distinction between being and essence in past entries, but I'd like to provide a brief overview of the argument.
Essence: what a thing is, e.g. its nature.
Being: that a thing is, e.g. its existence or actuality.
We know that the essence of a unicorn (roughly) is of a magical horse with a horn. However, that doesn't commit us to saying that a unicorn exists. The difference between a real unicorn and a not-real unicorn is that the former exemplifies being, even though the essence of each is relatively the same.
While there are many essences, there is only one being. For, to be distinct from being is to be non-being. Since anything with non-being simply doesn't exist, it follows that we are justified in deducing the unicity of being.
1. Something exists. (Premise)
2. Necessarily, something can exist if and only if being exists. (Definition)
3. Therefore, being exists. (From 1 and 2)
(2) is true by definition. If there is no being (existence), then nothing can possibly have the attribute of existence. I strongly disagree with Kant on this matter. If being/existence is not a thing, then a real unicorn and a non-real unicorn differ by a non-thing (literally, nothing), which is the same as saying that they are not distinct at all. Since this is absurd, it follows that being exists.
Since we have already established the unicity of being, what are some of the other divine attributes we can deduce?
Being must be eternal and omnipresent. After all, there is no time or place at which anything can exist apart from being. On the same token, being must be temporally necessary, by virtue of its necessary eternality.
Being must also be omnipotent. Since being is the efficient first cause of all essences, and because no effect is greater than its cause, it follows that being is capable of actualizing all potentialities that essences are able to actualize.
In sum, being exists, and being possesses the divine attributes of unicity, eternality, omnipresence, temporal necessity and omnipotence. This, as the Angelic Doctor muses, everyone understands to be God.