Objection 1. It seems that power is not in God. For as primary matter is to power, so God, who is the first agent, is to act. But primary matter, considered in itself, is devoid of all act. Therefore, the first agent--namely, God--is devoid of power.
Objection 2. Further, according to the Philosopher (Metaph. vi, 19), better than every power is its act. For form is better than matter; and action than active power, since it is its end. But nothing is better than what is in God; because whatsoever is in God, is God, as was shown above (Question 3, Article 3). Therefore, there is no power in God.
Objection 3. Further, Power is the principle of operation. But the divine power is God's essence, since there is nothing accidental in God: and of the essence of God there is no principle. Therefore there is no power in God.
Objection 4. Further, it was shown above (14, 8; 19, 4) that God's knowledge and will are the cause of things. But the cause and principle of a thing are identical. We ought not, therefore, to assign power to God; but only knowledge and will.
On the contrary, It is said: "Thou art mighty, O Lord, and Thy truth is round about Thee" (Psalm 88:9).
I answer that, Power is twofold--namely, passive, which exists not at all in God; and active, which we must assign to Him in the highest degree. For it is manifest that everything, according as it is in act and is perfect, is the active principle of something: whereas everything is passive according as it is deficient and imperfect. Now it was shown above (3, 2; 4, 1 and 2), that God is pure act, simply and in all ways perfect, nor in Him does any imperfection find place. Whence it most fittingly belongs to Him to be an active principle, and in no way whatsoever to be passive. On the other hand, the notion of active principle is consistent with active power. For active power is the principle of acting upon something else; whereas passive power is the principle of being acted upon by something else, as the Philosopher says (Metaph. v, 17). It remains, therefore, that in God there is active power in the highest degree.
Reply to Objection 1. Active power is not contrary to act, but is founded upon it, for everything acts according as it is actual: but passive power is contrary to act; for a thing is passive according as it is potential. Whence this potentiality is not in God, but only active power.
Reply to Objection 2. Whenever act is distinct from power, act must be nobler than power. But God's action is not distinct from His power, for both are His divine essence; neither is His existence distinct from His essence. Hence it does not follow that there should be anything in God nobler than His power.
Reply to Objection 3. In creatures, power is the principle not only of action, but likewise of effect. Thus in God the idea of power is retained, inasmuch as it is the principle of an effect; not, however, as it is a principle of action, for this is the divine essence itself; except, perchance, after our manner of understanding, inasmuch as the divine essence, which pre-contains in itself all perfection that exists in created things, can be understood either under the notion of action, or under that of power; as also it is understood under the notion of "suppositum" possessing nature, and under that of nature. Accordingly the notion of power is retained in God in so far as it is the principle of an effect.
Reply to Objection 4. Power is predicated of God not as something really distinct from His knowledge and will, but as differing from them logically; inasmuch as power implies a notion of a principle putting into execution what the will commands, and what knowledge directs, which three things in God are identified. Or we may say, that the knowledge or will of God, according as it is the effective principle, has the notion of power contained in it. Hence the consideration of the knowledge and will of God precedes the consideration of His power, as the cause precedes the operation and effect.
Source: Summa Theologica (New Advent)