Growing up made easier
God gives us a clue to His identity in Exodus:
"But," said Moses to God, "if I go to the Israelites and say to them, 'The God of your ancestors has sent me to you,' and they ask me, 'What is his name?' what do I tell them?"
God replied to Moses: I am who I am. Then he added: This is what you will tell the Israelites: I AM has sent me to you.
Catholics believe that God is existence ("I AM", that is, "to be" or "to exist"). From this, it is possible to deduce that He is omnipotent (all-powerful), omnipresent, eternal, and omniscient (all-knowing).1 Similarly, since God is what is, He is truth.
God's existence is objective: He either exists or He does not. He cannot exist for some people but not others. So how do we know He exists? Let's follow the reasoning set forth by St. Thomas Aquinas#Tertia_Via:_The_Argument_from_Contingency):
Why do things exist? That is, consider North America. North America exists, but it didn't have to. That is, it's contingent. North America wouldn't exist, for instance, if the earth didn't exist. But the earth also didn't have to exist; it, too, is contingent. The earth wouldn't exist, for instance, if it were not for the oxygen and silicon atoms that make up its crust. But those atoms are didn't have to exist either; they, too, are contingent. They would not exist, for instance, if it were not for subatomic particles.
We can continue in this vein until we come to the conclusion that everything in our universe exists, but none of it had to. That is, everything in the universe is contingent upon other things for their own existence. But we simply cannot have an infinite chain of contingencies. Why not? Because we haven't explained the existence of any item in the chain. For instance, suppose you have a train that's moving2. Each boxcar relies on the movement of the boxcar in front of it for its motion. Even if you have an infinitely long train, you will not have explained the movement of any of the boxcars. What you need instead is to come to a final element in the train upon which all the boxcars rely for their movement: the locomotive. Notice that the locomotive is of an entirely different species than the boxcars; it is capable of generating movement on its own and is not contingent on other cars in the train for its movement.
In the same way, we travel the chain of contingencies from North America through subatomic particles until we get to a final reality which is not contingent on anything in the universe for its existence: the very act of existence itself. Catholics call this God.
Certainly this does not prove God's love or His goodness; we will concern ourselves with those qualities later. But we see that God, who is existence itself, must not be contingent. He is another species altogether: a necessary being, without which nothing else in the universe would exist. In this way, we believe that God is reponsible for the universe; that is, He created it.
Certainly there are objections to this line of reasoning.
Even if the universe were infinitely old, it still exists even though it didn't have to. That is, it's still contingent and requires an explanation.
First, Stephen Hawking would propose that the laws of physics instead of God are responsible for the existence of matter:
I think the unierse was spontaneously created out of nothing, according to the laws of science.
The problem with this assertion is that Hawking hides another contingency in his solution: the laws of science. Indeed, the laws of science exist, but they don't explain their own existence. That is, why do the laws of science exist? Most scientists are apt to answer "they just are" and drop the whole issue. But the laws of science require an explanation, and they certainly do not explain themselves.3
The multiverse is the idea that there are parallel universes to ours. In each "universe", it is posited, there are different values for the physical constants and hence different scientific laws. Some of the universes aren't viable, but ours (by random chance) is. The odds of that happening might seem small, proponents argue, but with infinitely many parallel universes, one is bound to be viable like ours.
The problem with the multiverse is that it still fails to explain the existence of our universe; instead, it simply creates (infinitely many) more universes to explain.4
Stephen Hawking puts this objection this way:
When people ak me if a God created the universe, I tell them that the question itself makes no sense. Time didn't exist before the Big Bang so there is no time for God to make the universe in.
Hawking is right that asking what happens before time starts doesn't make sense. But God didn't do anything before time; rather, He is outside of time entirely. He created time.
We do not pretend that everything needs a cause. Only those things which begin to exist require a cause. That is to say, only contingent things require a cause. God, who is not contingent and who did not have a beginning, is a necessary being, not a contingent one. Therefore, nothing created God. As existence itself, He simply is.
God, who is existence, can do or create anything which can exist or which is logically consistent. That is to say, He cannot create contradictory things, such as a square circle; there is no such thing, nor can there ever be such a thing. Likewise, there can be no such thing as a rock so large that a God who can lift anything cannot lift it.5