The further conclusion follows that God is His own essence. The essence of anything is that which its definition signifies. This is identical with the thing of which it is the definition, unless per accidens something is added to the thing defined over and above its definition. Thus whiteness is added to man, over and above the fact that he is a rational and mortal animal. Hence rational and mortal animal is the same as man; but whiteness, so far as it is white, is not the same as man. In any being, therefore, in which there are not found two factors whereof one is per se and the other per accidens, its essence must be altogether identical with it. In God, however, since He is simple, as has been shown, there are not found two factors whereof one is per se and the other per accidens. Therefore His essence must be absolutely the same as He Himself.
Moreover, whenever an essence is not absolutely identical with the thing of which it is the essence, something is discerned in that thing that has the function of potency, and something else that has the function of act. For an essence is formally related to the thing of which it is the essence as humanity is related to man. In God. however, no potency and act can be discerned: He is pure act. Accordingly He is His essence.
St. Thomas Aquinas. (1265-1274). Compendium Theologiae: Identity of God with His Essence, trans. by Cyril Vollert. St. Louis & London: B. Herder Book Co., 1947
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