(CG 2/8). That God's Power is His Substance (Book 2: God The Origin of Creatures) (Summa Contra Gentiles) (Aquinas, Thomas, St.)

God's power is His substance, as has been shown in the previous chapter: also His action is His substance, as has been shown of His intellectual activity (B. I, Chap. XLV), and the same argument holds of His other activities. Therefore in God power and action are not two different things.

2. The action of any being is a complement of its power; for it stands to power as the second actuality to the first. But the divine power, being God's very essence, has no other complement than itself. And therefore in God action and power are not distinct.

Any action that is not the agent's very substance is in the agent as an accident in its subject. But in God there can be nothing accidental. Therefore in God His action is none other than His substance and His power.