2. Everything is by its own existence. Whatever then is not its own existence does not of itself necessarily exist. But God does of Himself necessarily exist: therefore God is His own existence.
4. 'Existence' denotes a certain actuality: for a thing is not said to 'be' for what it is potentially, but for what it is actually. But everything to which there attaches an actuality, existing as something different from it, stands to the same as potentiality to actuality. If then the divine essence is something else than its own existence, it follows that essence and existence in God stand to one another as potentiality and actuality. But it has been shown that in God there is nothing of potentiality (Chap. XVI), but that He is pure actuality. Therefore God's essence is not anything else but His existence.*
5. Everything that cannot be except by the concurrence of several things is compound. But nothing in which essence is one thing, and existence another, can be except by the concurrence of several things, to wit, essence and existence. Therefore everything in which essence is one thing, and existence another, is compound. But God is not compound, as has been shown (Chap. XVIII). Therefore the very existence of God is His essence. This sublime truth was taught by the Lord to Moses (Exod. iii, 13, 14) If they say to me, What is his name? what shall I say to them? Thus shalt thou say to the children of Israel: He who is hath sent me to you: showing this to be His proper name, He who is. But every name is given to show the nature or essence of some thing. Hence it remains that the very existence or being of God is His essence or nature.
1.21 : That God is His own Essence
1.23 : That in God there is no Accident