Summa contra Gentiles2.8 Chapter VIII. THAT god's power IS HIS SUBSTANCE

We may also conclude from this that the divine power is God's very substance.

For active power becomes a thing according as this is in act. Now God is very act; nor is He being in act by some act that is not Himself, since in Him there is no potentiality, as we have proved in the First Book. Therefore He is His own power.

Again. Whatever is powerful and is not its own power, is powerful by participating another's power. But nothing can be ascribed to God by participation, for He is His own being, as we proved in the First Book. Therefore He is His own power.

Moreover. Active power belongs to a thing's perfection, as stated above." Now every perfection of God is contained in His very being, as was shown in the First Book. Therefore the divine power is not other than His very being. Now God is His own being, as we proved in the First Book. Therefore He is His own power.

Again. In those things whose powers are not their substance, their powers are accidents: hence natural power is placed in the second species of accident. But in God there can be no accident, as was proved in the First Book. Therefore God is His own power.

Further. Whatever is by another is reduced to that which is by its very self, being thus reduced to that which is first. Now other agents are reduced to God as first agent.' Therefore He is agent by His very self. But that which acts by its very self, acts by its essence: and that by which a thing acts is its active power. Therefore God's very essence is His active power.