St Thomas Aquinas, in the first part of his Summa Theologica, after having enumerated five proofs for the existence of God, proceeds to outline and explicate his attributes, of which Thomas says He has eight:

1) Simplicity, 2) perfection, 3) goodness, 4) infinity, 5) ubiquity, 6) immutability, 7) eternity, and 8) unity.

This is the order in which he deals with them and I thought I would provide a quick summary of each in the same order for those that are interested.

To read what Thomas himself says, start here in the Summa.

1. THE SIMPLICITY OF GOD. The Simplicity of God means that God has no parts, that He is not composed in any way. He is not, as we are, the composition of body and soul, nor is He the composition of essence and existence. One of Thomas' arguments for why God is not a composition of body and soul is the following: 1. Bodies, by necessity, move. 2. God is the unmoved mover. 3. Therefore God does not have a body (this may be the quickest refutation of Mormonism ever!). Nor is God a composition of essence (what a thing is) and existence (that a thing is). Rather, in God, essence and existence are the same thing. We see Biblical evidence of this in the book of Exodus where God responds to Moses' request for a name, "I am who I am" (3:14). If essence and existence were not the same in God, if what he was was not the same thing as that he was, then there would exist outside of God the reason for his existence, which is absurd.

2. THE PERFECTION OF GOD

The perfection of God means that God lacks nothing. He is the the fullness of being. As St. Thomas says, "to be the first principle for others it is necessary to be maximally in act, and as such the most perfect being." The more a being is like God, the more perfect it is. The fact that we can even say that one being is more perfect than another implies a most perfect being, and this, of course, is the crux of Thomas' fourth proof for the existence of God.

3. THE GOODNESS OF GOD God, who is the greatest conceivable being, is also goodness itself, since being and goodness are really convertible. A being, in so far as it is like God, is good, and a being, in so far as it is unlike God, is not. Given that all creation is ordered to this good, one might reasonably ask, "If all creation is ordered toward the supreme good, who is God, to what good may we say that God is ordered? To none other than the supreme good who is himself. The only appropriate finality for an infinite being is infinite being."

4. THE INFINITY OF GOD The infinity of God refers to the fact that God is in no way limited. He is Subsistent being itself. "God is," in the words of D.Q. McInerny, "without limits because He is Himself the inexhaustible source of all the riches of being.

5. THE UBIQUITY OF GOD The ubiquity of God means that God is everywhere. Not that God has a body and is "in" each and every place—God is not inside of my glass of milk in the sense that he has extension in space. Rather, what we mean when we say God is everywhere—including my glass of milk—is this: "wherever something is operating, there it is." Since God is operating everywhere, he is everywhere. The sentiment "God is closer to you than you are to yourself," is not just a feel-good sentiment, it's true.

6. THE IMMUTABILITY OF GOD The sixth attribute put forward by Aquinas is His immutability, that is, his unchangeableness. If a thing changes it changes for better or for worse. If God was mutable, therefore, his changing would make him better or worse. If it made him better then he wasn't perfect to begin with. If it made him worse, then he isn't perfect now.

7. THE ETERNITY OF GOD The philosopher Boethius, in The Consolation of Philosophy defines eternity thusly: "Eternity is the everlasting, totally simultaneous and perfect possession of life." And this is the definition St. Thomas adopts when speaking of God's eternity. God is eternal because he immutable. "If we are right in describing time as the measure of motion," writes, McInerny, "we are equally right in describing eternity as the measure of permanence."

8. THE UNITY OF GOD Unity, like goodness, and beauty, is a transcendental of being. It is because God is being that he issimple that he is perfect unity. St. Thomas writes, "that which is simple, is undivided both with respect to act and potency." And since it is not possible to divide God in any way (matter/form; essence/existence, etc.), then it follows that God is perfect in unity.