Question 3 of Summa Theologica concerns "the simplicity of God." When we consider this question we are initially tempted to say (as we are of the Summa itself), "There's nothing simple about it!" That's because the medieval use of "simple" is different from today's usage. We think of simple as easy to grasp or understand, uncomplicated. They were thinking of oneness, or something that is a unity in itself. So, the question can be posed in terms of: "How is God one?" or, "How should we think about the oneness of God?"
He starts out by telling us that we cannot so much as know what or who God is, as we can think of it in terms of what he is not. He addresses the questions of whether God has a body, or if God is made up of matter and form together. The answer to both is no -- God is not corporal, and he is not matter, for he created it. He has a unique form, his essence; he is God and he is godly.
The question I am looking at here is (paraphrased): Are "essence" and "existence" the same thing in Who God is? Earlier, St. Thomas Aquinas has told us that God's essence and existence are the same thing, and we know this by his effects. For example, God is good to us (effect) because God is (existence) good (essence). Now he wants of dive deeper into why essence and existence are the same for God. I don't know if I've got all my ducks lined up in a row here; just think of it has mental gymnastics, and I probably won't get a 10. But these things are challenging to think about, and we all need to be challenged!
An objection of his to this is: God exists, but that is separate from what he is. Today, we are prone to separate the fact that God exists from his characteristics. We think more in terms of "God loves" than "God is love". God's very existence is viewed as just another of his traits. But that does not impress upon us the oneness of God. Although it is hard for us to do, we must think of God in greater terms than just what he does or what he is like. As God said to Isaiah, "I am the Lord, that is my name." (Isa.42:8) In response to this objection he gives a quote from St. Hilary, who opposed the Arian heresy in the 300's. "In God existence is not an accidental (non-substantial) quality, but subsisting truth." St. Thomas adds, "Therefore what subsists in God is his existence." St. Thomas then gives us 3 "proofs" for this concept.
1. In the case of things or people, existence is different from essence because it is either caused by an outside agent or it is an element of one's overall make-up. (The existence of heat is caused by fire; laughter is a part of humanness, he explains.) But, God is the First Cause of all existence outside of himself -- he is uncaused -- he simply is.
2. Existence makes things real, or, to use St. Thomas' word, "actual." Things are said to be real because they exist. Essence (the qualities of a thing) are part of its reality. If something that has essence is real, it must exist. (Apolo, my dog exists because he is a real dog. His doggyness is part of his reality. Apolo's doggyness is real (proven by his smell!), so he must exist!) In the world of things and people, existence can be real in a particular way (actual) or subject to change (potential). (Apolo is a certain age today, but he will get older.) But God, because his existence is (as in his name I AM), does not change; his reality is always constant.
3. Something (X) that is not something else (Y) may participate in (share in, be involved in) the something else (X in Y). But the existences are separate. A frog is in the water and it is wet, but the frog exists separate from the water. Also, the essences of a frog and water are different. But the water has an effect on the frog; it makes him wet. But God does not participate in anything else. He is singular, he is one, he is the beginning and the end, he is all in all.
So, God is God Who exists. Simple, but incomprehensible...