Objection 1. It seems that God is not omnipotent. For movement and passiveness belong to everything. But this is impossible with God, for He is immovable, as was said above (Question 2, Article 3). Therefore He is not omnipotent.
Objection 2. Further, sin is an act of some kind. But God cannot sin, nor "deny Himself" as it is said in 2 Timothy 2:13. Therefore He is not omnipotent.
Objection 3. Further, it is said of God that He manifests His omnipotence "especially by sparing and having mercy" [Collect, 10th Sunday after Pentecost]. Therefore the greatest act possible to the divine power is to spare and have mercy. There are things much greater, however, than sparing and having mercy; for example, to create another world, and the like. Therefore God is not omnipotent.
Objection 4. Further, upon the text, "God hath made foolish the wisdom of this world" (1 Corinthians 1:20), a gloss says: "God hath made the wisdom of this world foolish [Vulgate: 'Hath not God', etc.] by showing those things to be possible which it judges to be impossible." Whence it would seem that nothing is to be judged possible or impossible in reference to inferior causes, as the wisdom of this world judges them; but in reference to the divine power. If God, then, were omnipotent, all things would be possible; nothing, therefore impossible. But if we take away the impossible, then we destroy also the necessary; for what necessarily exists is impossible not to exist. Therefore there would be nothing at all that is necessary in things if God were omnipotent. But this is an impossibility. Therefore God is not omnipotent.
On the contrary, It is said: "No word shall be impossible with God" (Luke 1:37).
I answer that, All confess that God is omnipotent; but it seems difficult to explain in what His omnipotence precisely consists: for there may be doubt as to the precise meaning of the word 'all' when we say that God can do all things. If, however, we consider the matter aright, since power is said in reference to possible things, this phrase, "God can do all things," is rightly understood to mean that God can do all things that are possible; and for this reason He is said to be omnipotent. Now according to the Philosopher (Metaph. v, 17), a thing is said to be possible in two ways.
First in relation to some power, thus whatever is subject to human power is said to be possible to man.
Secondly absolutely, on account of the relation in which the very terms stand to each other. Now God cannot be said to be omnipotent through being able to do all things that are possible to created nature; for the divine power extends farther than that. If, however, we were to say that God is omnipotent because He can do all things that are possible to His power, there would be a vicious circle in explaining the nature of His power. For this would be saying nothing else but that God is omnipotent, because He can do all that He is able to do.
It remains therefore, that God is called omnipotent because He can do all things that are possible absolutely; which is the second way of saying a thing is possible. For a thing is said to be possible or impossible absolutely, according to the relation in which the very terms stand to one another, possible if the predicate is not incompatible with the subject, as that Socrates sits; and absolutely impossible when the predicate is altogether incompatible with the subject, as, for instance, that a man is a donkey.
It must, however, be remembered that since every agent produces an effect like itself, to each active power there corresponds a thing possible as its proper object according to the nature of that act on which its active power is founded; for instance, the power of giving warmth is related as to its proper object to the being capable of being warmed. The divine existence, however, upon which the nature of power in God is founded, is infinite, and is not limited to any genus of being; but possesses within itself the perfection of all being. Whence, whatsoever has or can have the nature of being, is numbered among the absolutely possible things, in respect of which God is called omnipotent. Now nothing is opposed to the idea of being except non-being. Therefore, that which implies being and non-being at the same time is repugnant to the idea of an absolutely possible thing, within the scope of the divine omnipotence. For such cannot come under the divine omnipotence, not because of any defect in the power of God, but because it has not the nature of a feasible or possible thing. Therefore, everything that does not imply a contradiction in terms, is numbered amongst those possible things, in respect of which God is called omnipotent: whereas whatever implies contradiction does not come within the scope of divine omnipotence, because it cannot have the aspect of possibility. Hence it is better to say that such things cannot be done, than that God cannot do them. Nor is this contrary to the word of the angel, saying: "No word shall be impossible with God." For whatever implies a contradiction cannot be a word, because no intellect can possibly conceive such a thing.
Reply to Objection 1. God is said to be omnipotent in respect to His active power, not to passive power, as was shown above (Article 1). Whence the fact that He is immovable or impassible is not repugnant to His omnipotence.
Reply to Objection 2. To sin is to fall short of a perfect action; hence to be able to sin is to be able to fall short in action, which is repugnant to omnipotence. Therefore it is that God cannot sin, because of His omnipotence. Nevertheless, the Philosopher says (Topic. iv, 3) that God can deliberately do what is evil. But this must be understood either on a condition, the antecedent of which is impossible--as, for instance, if we were to say that God can do evil things if He will. For there is no reason why a conditional proposition should not be true, though both the antecedent and consequent are impossible: as if one were to say: "If man is a donkey, he has four feet." Or he may be understood to mean that God can do some things which now seem to be evil: which, however, if He did them, would then be good. Or he is, perhaps, speaking after the common manner of the heathen, who thought that men became gods, like Jupiter or Mercury.
Reply to Objection 3. God's omnipotence is particularly shown in sparing and having mercy, because in this is it made manifest that God has supreme power, that He freely forgives sins. For it is not for one who is bound by laws of a superior to forgive sins of his own free will. Or, because by sparing and having mercy upon men, He leads them on to the participation of an infinite good; which is the ultimate effect of the divine power. Or because, as was said above (Question 21, Article 4), the effect of the divine mercy is the foundation of all the divine works. For nothing is due to anyone, except on account of something already given him gratuitously by God. In this way the divine omnipotence is particularly made manifest, because to it pertains the first foundation of all good things.
Reply to Objection 4. The absolute possible is not so called in reference either to higher causes, or to inferior causes, but in reference to itself. But the possible in reference to some power is named possible in reference to its proximate cause. Hence those things which it belongs to God alone to do immediately--as, for example, to create, to justify, and the like--are said to be possible in reference to a higher cause. Those things, however, which are of such kind as to be done by inferior causes are said to be possible in reference to those inferior causes. For it is according to the condition of the proximate cause that the effect has contingency or necessity, as was shown above (14, 1, ad 2). Thus is it that the wisdom of the world is deemed foolish, because what is impossible to nature, it judges to be impossible to God. So it is clear that the omnipotence of God does not take away from things their impossibility and necessity.
Source: Summa Theologica (New Advent)