“The Christian sense of God and the world was elaborated in the metaphysics of St. Thomas Aquinas. He speaks of God as ipsum esse subsistens and describes all other beings as existing through a participation in esse. Aquinas thinks of being in a context very much different from the final context Aristotle and Plato and other pagan philosophers acknowledged; Aquinas thinks of beings over against their sheer nonexistence, over against the nonexistence of the world or the whole, whereas pagan thinkers thought of things as being or not being within the whole. Aquinas considers the existence of things to be an actuality, an actuality determined by what the thing in each case is. Any given thing is not a sheer actuality, but an actuality of a special kind. The kind or essence lets the actuality occur, but it also confines the actuality to existing only in its particular way. The essence, as a potentiality to exist, allows something to be this sort of thing, but it also allows it to be only this sort of thing. Any particular being is actual and therefore has the perfection of existing, but it can be actual only because it must be contented to be within certain limits. Each thing is allowed to have its perfections because it is what it is, but the very possession of such perfections is at the same time the exclusion of the perfections of other kinds of things: a tree is not and cannot be a man, a dog is not and cannot be a diamond. This exclusion is characteristic of all ‘limited’ beings. Their act of existence, their esse is confined by what they are, by their essence, to being only this kind of existent.
“But God is the sheer act of esse subsistens, the sheer act of existing. He is not confined to being this kind of thing as opposed to that kind. He is not a ‘kind’ of thing at all, only sheer esse. Does the unqualified act of esse exclude the act of existing as a man? Or the act of existing as an animal? Or the act of existing as a tree? In the Thomistic understanding this kind of question is inappropriate. Between God and creatures there is no exclusion like th exclusion among finite beings. Creatures each exist in a certain way, whereas God is pure existence. Whatever goodness or greatness occurs in creatures occurs therefore in an eminent way in God. Hence after creation there are more beings but not more perfection of esse.”
-Fr. Robert Sokolowski, The God of Faith and Reason
A lucid and brief overview of the Thomistic perspective essence and existence in reference to God and man.