Objection 1. It seems that essence and existence are not the same in God. For if it be so, then the divine being has nothing added to it. Now being to which no addition is made is universal being which is predicated of all things. Therefore it follows that God is being in general which can be predicated of everything. But this is false: "For men gave the incommunicable name to stones and wood" (Wisdom 14:21). Therefore God's existence is not His essence.
Objection 2. Further, we can know "whether" God exists as said above (I:2:2); but we cannot know "what" He is. Therefore God's existence is not the same as His essence--that is, as His quiddity or nature.
On the contrary, Hilary says (Trin. vii): "In God existence is not an accidental quality, but subsisting truth." Therefore what subsists in God is His existence.
I answer that, God is not only His own essence, as shown in the preceding article, but also His own existence. This may be shown in several ways.
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.
Reply to Objection 1. A thing that has nothing added to it can be of two kinds. Either its essence precludes any addition; thus, for example, it is of the essence of an irrational animal to be without reason. Or we may understand a thing to have nothing added to it, inasmuch as its essence does not require that anything should be added to it; thus the genus animal is without reason, because it is not of the essence of animal in general to have reason; but neither is it to lack reason. And so the divine being has nothing added to it in the first sense; whereas universal being has nothing added to it in the second sense.
Reply to Objection 2. "To be" can mean either of two things. It may mean the act of essence, or it may mean the composition of a proposition effected by the mind in joining a predicate to a subject. Taking "to be" in the first sense, we cannot understand God's existence nor His essence; but only in the second sense. We know that this proposition which we form about God when we say "God is," is true; and this we know from His effects (I:2:2).
Source: Summa Theologica (New Advent)