To address this question, we must differentiate between
passive potency (which is the potency of matter) and
active potency (which is the potency of God, humans and other agents). Aquinas addressed this in the Summa:
Power [ie potency] 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. (STh, q.25 a.1)
Furthermore, God’s active potency is manifest in the temporal world, according to the traditional interpretation of John 5:17: “My Father continues to work.” Make sense?
-Ryan Vilbig
ryan.vilbig@gmail.com