"Omni" means all, so one can see immediately that God is all knowing, all powerful, all goodness, and all presence.
As "omnipotence" is the easiest to grasp, let me start with a quick view of this Attribute. Of course, this entire series is a horrible, short abbreviation, and mere introduction to the Church's teaching on God. I can only encourage readers to continue with studying Aquinas and his commentators.
The power of God is both logical and infinite. God cannot contradict Himself, which is a teaching wherein we separate ourselves from the second largest religion in the world. Here is Aquinas on the first part of his discussion on God's power.
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 actand 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.
Our imaginations cannot grasp the power of God as actual, as we live in a world of potential. God's Knowledge and Will are not separated from His power. All are part of His Divine Essence.
We are separated, divided in our beings with regard to power, knowledge, will, and action because of sin. Sin separates man's ability to be whole in a way God is Simplicity and Unity. Original sin destroyed this correlation of action, thought, decision, knowledge in all humans, only brought back into harmony through the grace of baptism.
That God is Infinite, another Attribute, allows us to understand His Omnipotence.
Here is the great Doctor again.
As stated above (Article 1), active power exists in God according to the measure in which He is actual. Now His existence is infinite, inasmuch as it is not limited by anything that receives it, as is clear from what has been said, when we discussed the infinity of the divine essence (7, 1). Wherefore, it is necessary that the active power in God should be infinite. For in every agent is it found that the more perfectly an agent has the form by which it acts the greater its power to act. For instance, the hotter a thing is, the greater the power has it to give heat; and it would have infinitepower to give heat, were its own heat infinite. Whence, since the divine essence, through which God acts, is infinite, as was shown above (Question 7, Article 1) it follows that His power likewise is infinite.
Reply to Objection 1. The Philosopher is here speaking of an infinity in regard to matter not limited by any form; and such infinity belongs to quantity. But the divine essence is otherwise, as was shown above (Question 7, Article 1); and consequently so also His power. It does not follow, therefore, that it is imperfect.
Reply to Objection 2. The power of a univocal agent is wholly manifested in its effect. The generative power of man, for example, is not able to do more than beget man. But the power of a non-univocal agent does not wholly manifest itself in the production of its effect: as, for example, the power of the sun does not wholly manifest itself in the production of an animal generated from putrefaction. Now it is clear that God is not a univocal agent. For nothing agrees with Him either in species or in genus, as was shown above (3, 5; 4, 3). Whence it follows that His effect is always less than His power. It is not necessary, therefore, that the infinite power of God should be manifested so as to produce an infinite effect. Yet even if it were to produce no effect, the power of God would not be ineffectual; because a thing is ineffectual which is ordained towards an end to which it does not attain. But the power of God is not ordered toward its effect as towards an end; rather, it is the end of the effect produced by it.
Reply to Objection 3. The Philosopher (Phys. viii, 79) proves that if a body had infinite power, it would cause a non-temporal movement. And he shows that the power of the mover of heaven is infinite, because it can move in an infinite time. It remains, therefore, according to his reckoning, that the infinite power of a body, if such existed, would move without time; not, however, the power of an incorporeal mover. The reason of this is that one body moving another is a univocal agent; wherefore it follows that the whole power of the agent is made known in its motion. Since then the greater the power of a moving body, the more quickly does it move; the necessary conclusion is that if its power were infinite, it would move beyond comparison faster, and this is to move without time. An incorporeal mover, however, is not a univocal agent; whence it is not necessary that the whole of its power should be manifested in motion, so as to move without time; and especially since it moves in accordance with the disposition of its will.
God is not limited in His power. His power has no limitations of time, the material, nor ordered to any particular end. Again, in the limitations of humans, one has difficulty imagining such a Being of Power Who Is Infinite. But, we can reason that such a God can exist and does.
Many modern people question God's ability to have infinite and complete, that is "all" power. A good discussion on this can be found here.
Aquinas writes:
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 isomnipotent, 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 helps if one has studied Logic here...
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 theangel, 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.
The second largest religion in the world gets tripped up on the above point. Theologians in that group think that God can contradict Himself, as contradiction is possible. But, the understandings of Perfection and what is possible are missing from their arguments. Their god is movable and contradictory.
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
Aquinas rightly shows a comparison to help us understand the difference in perfect and imperfect action..
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.
Many in today's world think that evil comes from God, or that the "evil force" is equal to God, as in neo-dualism.
And, as the Church moves to Mercy Sunday, God's omnipotence regarding mercy finds a place in our hearts, as well as in our minds.
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.
If you want to read about what God can do and not, look at the rest of this section for starters.Many agnostics and atheists get hung up on the relationship between God allowing us our free will and the presence of evil in the world. As we are made in God's image and likeness, and as satan was given free will himself, God allows the actions of will, even to the point of allowing, in his permissive will, evil.
That God does not want evil seems clear, but that His omnipotence allows for freedom strikes some people as a lack of power. I shall come back to this discussion in the next post.
http://www.newadvent.org/summa/1025.htm
Next post will cover, again painfully briefly, Omniscience.