Objection 1. It seems that God cannot do other than what He does. For God cannot do what He has not foreknown and pre-ordained that He would do. But He neither foreknew nor pre-ordained that He would do anything except what He does. Therefore He cannot do except what He does.
Objection 2. Further, God can only do what ought to be done and what is right to be done. But God is not bound to do what He does not; nor is it right that He should do what He does not. Therefore He cannot do except what He does.
Objection 3. Further, God cannot do anything that is not good and befitting creation. But it is not good for creatures nor befitting them to be otherwise than as they are. Therefore God cannot do except what He does.
On the contrary, It is said: "Thinkest thou that I cannot ask My Father, and He will give Me presently more than twelve legions of angels?" (Matthew 26:53). But He neither asked for them, nor did His Father show them to refute the Jews. Therefore God can do what He does not.
I answer that, In this matter certain persons erred in two ways. Some laid it down that God acts from natural necessity in such way that as from the action of nature nothing else can happen beyond what actually takes place--as, for instance, from the seed of man, a man must come, and from that of an olive, an olive; so from the divine operation there could not result other things, nor another order of things, than that which now is. But we showed above (Question 19, Article 3) that God does not act from natural necessity, but that His will is the cause of all things; nor is that will naturally and from any necessity determined to those things. Whence in no way at all is the present course of events produced by God from any necessity, so that other things could not happen. Others, however, said that the divine power is restricted to this present course of events through the order of the divine wisdom and justice without which God does nothing. But since the power of God, which is His essence, is nothing else but His wisdom, it can indeed be fittingly said that there is nothing in the divine power which is not in the order of the divine wisdom; for the divine wisdom includes the whole potency of the divine power. Yet the order placed in creation by divine wisdom, in which order the notion of His justice consists, as said above (Question 21, Article 2), is not so adequate to the divine wisdom that the divine wisdom should be restricted to this present order of things. Now it is clear that the whole idea of order which a wise man puts into things made by him is taken from their end. So, when the end is proportionate to the things made for that end, the wisdom of the maker is restricted to some definite order. But the divine goodness is an end exceeding beyond all proportion things created. Whence the divine wisdom is not so restricted to any particular order that no other course of events could happen. Wherefore we must simply say that God can do other things than those He has done.
Reply to Objection 1. In ourselves, in whom power and essence are distinct from will and intellect, and again intellect from wisdom, and will from justice, there can be something in the power which is not in the just will nor in the wise intellect. But in God, power and essence, will and intellect, wisdom and justice, are one and the same. Whence, there can be nothing in the divine power which cannot also be in His just will or in His wise intellect. Nevertheless, because His will cannot be determined from necessity to this or that order of things, except upon supposition, as was said above (Question 19, Article 3), neither are the wisdom and justice of God restricted to this present order, as was shown above; so nothing prevents there being something in the divine power which He does not will, and which is not included in the order which He has place in things. Again, because power is considered as executing, the will as commanding, and the intellect and wisdom as directing; what is attributed to His power considered in itself, God is said to be able to do in accordance with His absolute power. Of such a kind is everything which has the nature of being, as was said above (Article 3). What is, however, attributed to the divine power, according as it carries into execution the command of a just will, God is said to be able to do by His ordinary power. In this manner, we must say that God can do other things by His absolute power than those He has foreknown and pre-ordained He would do. But it could not happen that He should do anything which He had not foreknown, and had not pre-ordained that He would do, because His actual doing is subject to His foreknowledge and pre-ordination, though His power, which is His nature, is not so. For God does things because He wills so to do; yet the power to do them does not come from His will, but from His nature.
Reply to Objection 2. God is bound to nobody but Himself. Hence, when it is said that God can only do what He ought, nothing else is meant by this than that God can do nothing but what is befitting to Himself, and just. But these words "befitting" and "just" may be understood in two ways: one, in direct connection with the verb "is"; and thus they would be restricted to the present order of things; and would concern His power. Then what is said in the objection is false; for the sense is that God can do nothing except what is now fitting and just. If, however, they be joined directly with the verb "can" (which has the effect of extending the meaning), and then secondly with "is," the present will be signified, but in a confused and general way. The sentence would then be true in this sense: "God cannot do anything except that which, if He did it, would be suitable and just."
Reply to Objection 3. Although this order of things be restricted to what now exists, the divine power and wisdom are not thus restricted. Whence, although no other order would be suitable and good to the things which now are, yet God can do other things and impose upon them another order.
Source: Summa Theologica (New Advent)