Power, in God, is not in any sense passive, but active in the highest degree. For anything suffers the action of another, inasmuch as itself is deficient and imperfect; but God is Pure Actuality, simple and perfect; hence it eminently belongs to Him to be the Origin, and not to be in any way the recipient.
The power of God is infinite, because His Being is infinite, and in no way limited; for such is His Essence: and the power of action in any agent is proportioned to the perfection of the form by which it acts. Hence God is universally acknowledged as Omnipotent; for His Divine Being contains in Itself the perfection of all being, nor is anything repugnant to It except what would imply being and not being simultaneously; and this not from defect in the Divine Power, but because such things cannot be conceived. It is better, therefore, to say, of such things, that they cannot be done than to say that God cannot do them, e.g. to cause that the past should not have been does not fall under the power of God, because it implies a contradiction of being and not being, and consequently fails of conception as possible.
Since the Divine Wisdom is not determined to one order of things, God can do things other than He does. For His Goodness is an end so out of proportion to created things that it exceeds them infinitely, and from it the course of things may always flow differently. Hence God may do other things than those which He does, but He cannot do these better than they are done, if we speak of that goodness which pertains to the essence of things, as reason to the essence of man. It is otherwise, however, if we speak of a goodness outside the essence of things, as it is the good of man to be virtuous and wise; for in this sense God could make things better than those He has made. Absolutely speaking, therefore, whatever things God has made. He could make others better.
- text taken from Compendium of the Summa Theologica of Saint Thomas Aquinas, by Bishop Berardus Bongiovanni