Paraphrasing the Summa: First part, Question 2, Article 3

Question 2. The existence of God

Does God exist?

Article 3. Whether God exists?

Is there a God?

Objection 1. It seems that God does not exist; because if one of two contraries be infinite, the other would be altogether destroyed. But the word "God" means that He is infinite goodness. If, therefore, God existed, there would be no evil discoverable; but there is evil in the world. Therefore God does not exist.

It doesn't seem logical that God should exist.  Because if God is all good and God is omnipresent, that is to say, everywhere, then evil should not exist.  But we know that evil does exist.  Therefore, there can be no such thing as an omnipresent God. 

Objection 2. Further, it is superfluous to suppose that what can be accounted for by a few principles has been produced by many. But it seems that everything we see in the world can be accounted for by other principles, supposing God did not exist. For all natural things can be reduced to one principle which is nature; and all voluntary things can be reduced to one principle which is human reason, or will. Therefore there is no need to suppose God's existence.

Anyway, there is no need to imagine the existence of God because the existence of the world and all of the things in it can be accounted for in other ways. 

On the contrary, It is said in the person of God: "I am Who am." (Exodus 3:14)

St. Thomas counters,  Scripture says that God is He who is.  In other words, God is all which exists. 

*Of course, this is proof only for those who presuppose the existence of God and also admit the inspiration of Scripture.

I answer that, The existence of God can be proved in five ways.

There are five ways to prove the existence of God. 

The first and more manifest way is the argument from motion.

The first of the five and the easiest to visualize, is the argument from motion. 

It is certain, and evident to our senses, that in the world some things are in motion.

When we look at nature, we can see that some things are moving.  The moon, for instance.

Now whatever is in motion is put in motion by another,

And we know that nothing moves unless something moves it.  When we move, for example, it is because we move our bodies.   

for nothing can be in motion except it is in potentiality to that towards which it is in motion; whereas a thing moves inasmuch as it is in act. For motion is nothing else than the reduction of something from potentiality to actuality.

I think, by the term "actuality", he means, "action".  Therefore, motion is nothing but changing from the state of "potential action" to "action". 

But nothing can be reduced from potentiality to actuality, except by something in a state of actuality. Thus that which is actually hot, as fire, makes wood, which is potentially hot, to be actually hot, and thereby moves and changes it.

So, wood doesn't burn unless it is set on fire by something which is already hot enough to make it burn.

Now it is not possible that the same thing should be at once in actuality and potentiality in the same respect, but only in different respects. For what is actually hot cannot simultaneously be potentially hot; but it is simultaneously potentially cold. It is therefore impossible that in the same respect and in the same way a thing should be both mover and moved, i.e. that it should move itself. Therefore, whatever is in motion must be put in motion by another. If that by which it is put in motion be itself put in motion, then this also must needs be put in motion by another, and that by another again. But this cannot go on to infinity, because then there would be no first mover, and, consequently, no other mover; seeing that subsequent movers move only inasmuch as they are put in motion by the first mover; as the staff moves only because it is put in motion by the hand. Therefore it is necessary to arrive at a first mover, put in motion by no other; and this everyone understands to be God.

So, since we know that nothing moves unless something else moves it, we can trace back the action to the first mover.  And we know that He who moved first, is God.

This is the first way to prove that God exists.

The second way is from the nature of the efficient cause. In the world of sense we find there is an order of efficient causes. There is no case known (neither is it, indeed, possible) in which a thing is found to be the efficient cause of itself; for so it would be prior to itself, which is impossible. Now in efficient causes it is not possible to go on to infinity, because in all efficient causes following in order, the first is the cause of the intermediate cause, and the intermediate is the cause of the ultimate cause, whether the intermediate cause be several, or only one. Now to take away the cause is to take away the effect. Therefore, if there be no first cause among efficient causes, there will be no ultimate, nor any intermediate cause. But if in efficient causes it is possible to go on to infinity, there will be no first efficient cause, neither will there be an ultimate effect, nor any intermediate efficient causes; all of which is plainly false. Therefore it is necessary to admit a first efficient cause, to which everyone gives the name of God.

To me, this second proof is very similar to the first, except it is the case of the Maker.  The one who caused all things to come into existence is God.  But the proof goes to tracing back all things which were created to the first thing.  The First Thing could not have been created.  He is the Maker of first creatures and gave them the ability to reproduce and continue reproducing themselves until now. 

The third way is taken from possibility and necessity, and runs thus.

The third way to prove the existence of God is from reasoning that which is nonessential and that which is essential. 

We find in nature things that are possible to be and not to be, since they are found to be generated, and to corrupt, and consequently, they are possible to be and not to be.

In life, we see that things are here today and gone tomorrow.  They are born, age and die.  They are made, wax old and wear out.   These things are nonessential to the existence of the world.  Because the world was here before they existed and will remain after they pass.

But it is impossible for these always to exist, for that which is possible not to be at some time is not. Therefore, if everything is possible not to be, then at one time there could have been nothing in existence.

But, if everything were nonessential, then it would be possible that at one time, nothing existed at all.  

Now if this were true, even now there would be nothing in existence, because that which does not exist only begins to exist by something already existing.

But, if this were true, nothing today could exist.  Because nothing from nothing is nothing.  In order for something to come into existence, something else must bring it into existence.

Therefore, if at one time nothing was in existence, it would have been impossible for anything to have begun to exist; and thus even now nothing would be in existence — which is absurd.

Therefore, since nothing from nothing is nothing, it is ridiculous to believe that there ever was a time when something didn't exist. 

Therefore, not all beings are merely possible, but there must exist something the existence of which is necessary.

Therefore, there must be something which exists which is essential for the existence of everything else. 

But every necessary thing either has its necessity caused by another, or not. Now it is impossible to go on to infinity in necessary things which have their necessity caused by another, as has been already proved in regard to efficient causes. Therefore we cannot but postulate the existence of some being having of itself its own necessity, and not receiving it from another, but rather causing in others their necessity. This all men speak of as God.

Therefore, God must exist.   

*The rest of that paragraph is a brief summary of the first cause proof above (i.e. the second proof).

The fourth way is taken from the gradation to be found in things.

The fourth proof is from degree of things to be found.   

Among beings there are some more and some less good, true, noble and the like.

All things differ from one another to some degree.  Some are better, some are worse.   

But "more" and "less" are predicated of different things, according as they resemble in their different ways something which is the maximum, as a thing is said to be hotter according as it more nearly resembles that which is hottest; so that there is something which is truest, something best, something noblest and, consequently, something which is uttermost being; for those things that are greatest in truth are greatest in being, as it is written in Metaph. ii.

But everything is better or worse in comparison to that which is best.  There is always a standard to which everything is compared.

Now the maximum in any genus is the cause of all in that genus; as fire, which is the maximum heat, is the cause of all hot things. 

*Now, we don't normally think of God as hot.  Because we have been conditioned to think of hell as hot.  But, remember that Scripture says, "Hebrews 12:29; For our God is a consuming fire."

Therefore there must also be something which is to all beings the cause of their being, goodness, and every other perfection; and this we call God.

Therefore, there has to be something that can pass on all good things.  That being is He whom we call, God.

The fifth way is taken from the governance of the world.

The fifth proof is from Intelligent design.

We see that things which lack intelligence, such as natural bodies, act for an end, and this is evident from their acting always, or nearly always, in the same way, so as to obtain the best result. Hence it is plain that not fortuitously, but designedly, do they achieve their end.

We can look at all the things in our world and see that there is a grand design.

Now whatever lacks intelligence cannot move towards an end, unless it be directed by some being endowed with knowledge and intelligence; as the arrow is shot to its mark by the archer. Therefore some intelligent being exists by whom all natural things are directed to their end; and this being we call God.

But if there is no intelligence, then there can be no design.  And the more grand the design, the more grand the Designer.  Therefore, we know that there is One who designed all things.  We call Him, God.

Reply to Objection 1. As Augustine says (Enchiridion xi): "Since God is the highest good, He would not allow any evil to exist in His works, unless His omnipotence and goodness were such as to bring good even out of evil." This is part of the infinite goodness of God, that He should allow evil to exist, and out of it produce good.

Now, the first objection said that evil exists, therefore God can not exist.  However, St. Augustine says that God permits evil in order to bring about a greater good.

*I say, "look at the Crucifixion".  God permitted the most heinous evil, the Crucifixion, which is, essentially, men murdering God.  And He permitted that in order to bring about the redemption of all mankind and the salvation of those who repent of their sins.

Reply to Objection 2. Since nature works for a determinate end under the direction of a higher agent, whatever is done by nature must needs be traced back to God, as to its first cause. So also whatever is done voluntarily must also be traced back to some higher cause other than human reason or will, since these can change or fail; for all things that are changeable and capable of defect must be traced back to an immovable and self-necessary first principle, as was shown in the body of the Article.

The second objection said that can be many ways that creatures could have come into being.  Therefore, the idea of a grand creator is not necessary.

St. Thomas says that this is disproved in the five proofs above.  Just because men can concoct reasons for the existence of the universe, it doesn't mean they are right.  It just means they deny the truth.