You may occasionally come across the self-assured atheist or agnostic who believes he can prove to you that belief in God is an illogical and untenable position based on one or both of these two questions:
If God is all-powerful, can God make a rock so big even He can't lift it?
If everything needs a creator, who created God?
Briefly, I will show that these are damn silly questions that rely on basic logical errors for their rhetorical potency.
First: If God is all-powerful, can God make a rock so big even He can't lift it?
The argument goes like this: "You say you believe in a God who's omnipotent, who can do anything, who creates the moon and the stars and everything there is. Well, can God make a rock so big even He can't lift it? No? Then He can't do everything. Your 'all-powerful God' doesn't exist!"
Oh, gee, what a good point, you really got me there--NOT!
Here's the problem: the question contradicts itself. It is nonsense. Literally nonsense. It doesn't mean anything.
This seemingly clever question violates the most basic rule of logic, the principle of non-contradiction: "a thing cannot both be and not be at the same time and in the same respect." I cannot both be in the room and not be in the room at the same time and in the way same way. He cannot both be eating a cheeseburger and not eating a cheeseburger at the same time and in the same sense. Everybody and their great aunt Sylvia knows this; it's the foundation of our ability to think.
This question violates that principle in at least two ways. First, it assumes God's omnipotence ("If God is all-powerful") but then denies it by denying that he can do something. Second, the "something" it denies He can do is itself meaningless: there is no such thing as a rock so big that an all-powerful being couldn't lift it. To be all-powerful is to have every power, every ability, which would include the ability to lift anything, right? But it does not include the ability to make something that a being with the ability to lift anything is unable to lift. That's nonsense. There can be no such thing as "the ability to lift an un-lift-able rock." It's a self-contradictory definition. You might as well ask if God can make "a square circle," "a living dead thing," or "a dog that is a cat."
It's not a "gotcha" moment, or an unanswerable argument--well, perhaps it's unanswerable only in the sense that you can't answer a question with no meaning. I'm reminded of a Laurel and Hardy bit where someone asks the boys, "Lovely weather we are having tomorrow, wasn't it?" Ollie tries to answer, but realizes the question is ridiculous: it mixes past, present, and future tenses, and can't refer to any one time. The atheist's/agnostic's question here makes just as much sense.
On to the second question: If everything needs a creator, who created God?
The believer will argue something like, "Look, the world didn't just spring out of nothingness. Everything we see depends on something else outside of itself for its existence: babies come from parents, helium is formed by hydrogen fusing in stars, swords are made by swordsmiths. None of these things can account for its own existence. Everything depends on something else for its existence; everything in the world is contingent on something else. So there must be something that can account for everything existence, something that created it all, which itself is not contingent. And that must be God."
And the atheist/agnostic will smirk and reply, "Oh yeah? If everything needs a creator, then doesn't God, too? So who created God? And who created who created God? Huh?"
And the informed believer will reply: "Ah, I see, either you misunderstood, or I left something out. I said that everything we see in the world is contingent, it doesn't spring out of nowhere or cause itself. I mean by that: it doesn't have within itself the explanation for its existence; it depends on something else; it's contingent. So where did they come from? If we try to explain the existence of one contingent thing by the existence of another--babies come from parents, who come from parents, who come from parents, etc.--we get an infinite regress. We never come to the point where things began. And all things have a beginning, as we see with everything we encounter in the world. The only way to not have that infinite chain backward, the only way to have a starting point from which everything begins, is to have a First Cause, something that exists that doesn't depend on anything else for its existence--not a self-caused being so much as a non-contingent being, a necessary being, a being which has and does and will always exist, because its very nature is to exist. This First Cause or necessary being we call God. Only contingent beings need a creator. A necessary being does not. So God does not need a creator."
The mistake here is to think of God as one just one other existing thing among other existing things, even if He's the biggest and most powerful and way awesome-est thing there is. That's a mistake that will get you in a whole heap of philosophical trouble (as the late medieval nominalists and their modern progeny discovered, but that's for another time). God is not the biggest being among other beings. God is the very foundation of being. If the universe were a drawing on a chalkboard, God wouldn't be the biggest drawing of all, or the sum total of all the drawings: God would be the hand drawing on the board (possibly also the chalk, too, depending on how we take the analogy, but anyway....)
OK, I think I've packed too much into that last bit, but the point is: if ever someone faces you with these questions, give a gentle and charitable chuckle and explain to them the errors in their thinking. Don't let them fool you into thinking your faith is unreasonable or nonsensical. Introduce them to these arguments, and they'll soon discover the depth of logic to be found in the faith. After all, a belief must be logical when it is founded upon the Logos Himself.