Many people have said that the most important philosophical question is, "Why is there something rather than nothing?" This has often been repeated as if it is an obviously profound and fundamental question.
But actually, the question is absurd. It is absurd because it is a necessary truth that "being exists" and that "non-being doesn't exist." The words "being" or "something" simply mean "that which exists." We can rephrase the question to bring out the absurdity clearly: "Why is the state of being that exists a state of being that exists rather than a state of non-being which does not exist?" If you find this question to be profound, you should also find this question to be profound: "Why is a cookie a cookie and not a non-cookie?" The answer is, a cookie is a cookie because it is . . . well, a cookie. And being (that is, "that which exists") exists because . . . well, because it exists. What else could "being" do but exist? And why do we find it so profound and surprising that "nothing" doesn't exist? I mean, what did we expect it to be doing?
Understanding the fact that being necessarily exists and non-being necessarily does not exist is helpful in understanding a classic argument for the existence of God that has perplexed many people (including myself) over the years: the ontological argument. The ontological argument is basically an attempt to show that God exists by definition, that if you understand what "God" means, you will not be able to deny his existence.
A lot of people find it odd that one can prove something so important by means of a simple defining of terms; but, nevertheless, that is the way the world works sometimes. Starting with being rather than God can help to make the ontological argument more clear. Being necessarily exists; it exists by definition. If you understand what being means, you know that it exists. If you think you understand what being means but you are not sure that it must exist, you do not understand what you think you understand. ("You keep using that word. I don't think it means what you think it means.") The fact that it exists is built into the very concept of being and is inseparable from it.
Now, God is nothing other than Ultimate Being. That is, God is the name we give to "Complete and Utter Being" I am not Complete and Utter Being, and neither are you. Hopefully you've realized that by now. No finite object in the universe can be Complete and Utter Being, because any such object simply does not encompass within its own being all of being. It is a part of a larger whole. The entire space-time universe itself cannot be Complete and Utter Being either, because it has properties (such as being in time, being divisible, etc.) which require it to have been derived from a more ultimate state of being. (A temporal universe requires a starting point outside of itself; a divisible universe requires an origin in something that provides the unity of all its parts; etc. See the chapter on God in my book, Why Christianity is True, for more on these kinds of arguments.) Only a being like God has the necessary characteristics to be Complete and Utter Being, or Ultimate Being. To say that Ultimate Being does not exist is just as absurd as saying that being in general does not exist, for it would be to say that parts of being exist but not the whole of being. To say that the ultimate state of reality isn't real is a contradiction in terms. Therefore, God, by definition, exists.
Of course, the difficulty with the ontological argument is that in order to use it fully, you have to bring in other arguments showing why God and not, say, the universe or some impersonal thing must be Ultimate Reality, and so it really cannot stand on its own unless a lot of intuitive jumps are made. But as those other arguments are quite valid and sound, the ontological argument is valid and sound. God, by definition, exists; and no one can say "God does not exist" without involving himself in self-contradictory absurdity.