Thinking cap times, folks. What God is and that He is are the same thing. I AM WHO AM.
Chapter 22: That in God existence and essence are the same
1 FROM what has been shown above, we may go on to prove that in God essence or quiddity is not distinct from His existence.
2For it has been shown above that there is a thing which exists of itself necessarily, and this is God.i Now necessary existence, if it belong to a quiddity which is not that existence itself, is either inconsistent with or repugnant to that quiddity, as per se existence is to the quiddity of whiteness, or else is consistent or akin thereto, for instance that whiteness exist in some other thing. In the former supposition it will not belong to that quiddity to exist per se necessarily, for instance it becomes not whiteness to exist per se. In the second hypothesis, either this existence must be dependent on the essence, or both of them on some other cause, or the essence on the existence. The first two are in contradiction with the very notion of necessary per se existence: for if it depend on something else, it no longer exists necessarily. From the third supposition it follows that this quiddity is added accidentally to the thing which exists per se necessarily: because whatever follows on the essence of a thing is accidental thereto. Therefore God has not an essence distinct from His existence.ii
5…Further. Each thing exists by its own existence. Wherefore that which is not its own existence does not exist per se necessarily. But God exists per se necessarily. Therefore God is His own existence.
7…Moreover. Existence denotes a kind of actuality: since a thing is said to exist, not through being in potentiality, but through being in act. Now everything to which an act is becoming, and which is distinct from that act, is related thereto as potentiality to act: since act and potentiality are reciprocal terms. Accordingly, if the divine essence is distinct from its existence, it follows that His essence and existence are mutually related as potentiality and act. Now it has been proved that in God there is nothing of potentiality, and that He is pure act. Therefore God’s essence is not distinct from His existence.iii
9…Further. Everything exists through having existence. Therefore nothing the essence of which is not its existence, exists by its essence, but by participation of something, namely existence. Now that which exists by participation of something cannot be the first being, because that in which a thing participates in order to exist, is previous to that thing. But God is the first being, to which nothing is previous. Therefore God’s essence is His existence.iv
10 This sublime truth Moses was taught by the Lord: for when he asked the Lord (Exod. iii. 13, 14): If the children of Israel should say to me: What is His name? what shall I say to them? the Lord answered: I AM WHO AM….Thus shalt thou say to the children of Israel: HE WHO IS hath sent me to you; thus declaring His own name to be: HE WHO IS. Now every name is appointed to signify the nature or essence of a thing. Wherefore it follows that God’s very existence itself is His essence or nature.v
iChapter 13. Always Chapter 13. Go and re-read it. Don’t be lazy.
iiRepeat that paragraph three times fast. It’s all there, but not put in the friendliest language to us, whose gray matter is accidentally much smaller than Thomas’s. Considering that paragraph’s density is what might have led him to re-write that proof in a happier way, in the Summa Theologica, Book 1, Question 3, Article 4, sed contra:
First, whatever a thing has besides its essence must be caused either by the constituent principles of that essence (like a property that necessarily accompanies the species–as the faculty of laughing is proper to a man–and is caused by the constituent principles of the species), or by some exterior agent–as heat is caused in water by fire. Therefore, if the existence of a thing differs from its essence, this existence must be caused either by some exterior agent or by its essential principles. Now it is impossible for a thing’s existence to be caused by its essential constituent principles, for nothing can be the sufficient cause of its own existence, if its existence is caused. Therefore that thing, whose existence differs from its essence, must have its existence caused by another. But this cannot be true of God; because we call God the first efficient cause. Therefore it is impossible that in God His existence should differ from His essence.
Secondly, existence is that which makes every form or nature actual; for goodness and humanity are spoken of as actual, only because they are spoken of as existing. Therefore existence must be compared to essence, if the latter is a distinct reality, as actuality to potentiality. Therefore, since in God there is no potentiality, as shown above (Article 1), it follows that in Him essence does not differ from existence. Therefore His essence is His existence.
Thirdly, because, just as that which has fire, but is not itself fire, is on fire by participation; so that which has existence but is not existence, is a being by participation. But God is His own essence, as shown above (Article 3) if, therefore, He is not His own existence He will be not essential, but participated being. He will not therefore be the first being–which is absurd. Therefore God is His own existence, and not merely His own essence.
Admit it: that is some pretty writin’! “[N]othing can be the sufficient cause of its own existence, if its existence is caused.” God is not caused, He is the necessary being, as we already learned. Thomas also refers to Article 3, “Whether God is the same as His essence or nature?”, in which he notes that in things composed of matter and form (like us and baseball bats) essence and existence differ, but in things “in which individualization is not due to individual matter”, and since God is not made of matter but does have an essence and since forms or universals don’t exist materially (we are not Plantonists here), God “must be His own Godhead, His own Life, and whatever else is thus predicated of Him.” What is predicated of Him, we are still to learn (Chapter 30).
Now St Thomas continues in Chapter 22, and so shall we, but omitting some of the now redundant material.
iiiThis is redundant, but I like it, because we ever need to emphasize the difference between potentiality and actuality.
ivBy being “first” being, Thomas does not mean God was caused. God’s existence is necessary. It’s not that everything else came “after” God, but that God is necessary for the existence of everything else. God is outside time.
v(Ellipsis original.) We don’t often include scriptural evidence, and even here we are not asking you to rely on it as a separate proof that essence = existence (we already have enough proof). But just look at what this nearly illiterate pre-computerized pre-Enlightenment “desert tribe” came up with! We have deduced from first principles the very same name of God as they knew! I AM = existence. WHO AM = essence. I AM WHO AM is equivalent to existence = essence, and so is HE WHO IS. Ain’t you amazed?! (I’m speaking in the voice of Captain Aubrey.) Ch. xiii.
 Ch. xviii.
 Ch. xiii.
 Ch. xvi.
 Ch. xviii.
 Ch. xiii.
 vii. 11.