Summa contra Gentiles2.9 Chapter IX. THAT god's power IS HIS ACTION

From this we can show that God's power is not other than His action.

For things that are identical with one and the same thing, are identical with one another. Now God's power is His substance, as we have proved: and His action is also His substance, as we showed in the First Book with regard to His intellectual operation: for this applies equally to His other operations. Therefore in God power is not distinct from action.

Again. The action of a thing is a complement of its power: for it is compared to power as second act to first. Now the divine power is not completed by another than Himself, since it is God's very essence.'' Therefore in God power is not distinct from action.

Moreover. Just as active power is something acting, so is its essence something being. Now God's power is His essence, as we have proved. Therefore His action is His being. But His being is His substance." Therefore God's action is His substance, and so the same conclusion follows as before.

Further. An action that is not the substance of the agent, is in the agent as an accident in its subject: wherefore action is reckoned among the nine predicaments of accident. Now there can be nothing accidental in God.' Therefore God's substance is not other than His power.