You believe that there is no God? Well, what is God? Suppose that God is the greatest being that can be conceived. Now even if you don’t think that such a being exists, you can still understand the idea of such a being; you can still conceive of it. Therefore, whether or not such a being exists in reality, it must at least exist in your mind. But a being that existed in reality would be greater than a being that existed only in your mind. Therefore, for such a being to exist only in your mind, and not in reality, would be a contradiction in terms; for if it existed only in your mind, it would both be the greatest being that can be conceived (that’s what you’re conceiving of) and not be the greatest thing that can be conceived (because the same being existing in reality would be greater). So the greatest being that can be conceived – this being must exist in reality as well as in thought.
This is a simplified version of Anselm’s argument for the existence of God, often called the “ontological” argument. I’m not sure whether it really works; I’m inclined to say it doesn’t, although it’s hard to say where the logic goes wrong, especially in the more sophisticated version presented by Anselm himself.
Nevertheless, I consider it the best and most important of the proofs of God’s existence, even if it doesn’t work. Why? Because the God that it’s proving is actually God. A “greatest possible being” actually is likely to be omnipotent and omnibenevolent, a being that watches over us, intervenes in our affairs, and can tell us how we should and shouldn’t live our lives.
By contrast, most arguments for God’s existence are some variant on “cosmological” or “design” arguments – claiming only that the universe must be created by some intelligent entity or, more sparsely yet, some first cause that caused everything else to happen. But so what? If these arguments work, they don’t really matter all that much. The so-called God that they prove could just be some big thing that puts everything in motion and then goes away and ignores us, a “Divine Watchmaker.” And I don’t see that that makes any significant difference to our lives now. For that matter, they say nothing about the being’s goodness: Iris Murdoch quips that “a demon could have created the world.”
If Anselm’s argument works, it changes everything; if the cosmological or design arguments work, they change nothing, or almost nothing. Anselm’s argument doesn’t prove that God must have the historically specific qualities attributed to Him in any given tradition (such as becoming human in the person of Jesus); the God it proves could certainly be that worshipped by Jews, Muslims or Baha’is. But whichever tradition we associate Him with, this God is going to turn out a being that’s the best we can imagine – the most just, the most kind, the most powerful – and that’s the God whose existence, or lack of existence, actually matters.
EDIT: the last sentence of the first paragraph originally said “in thought as well as in reality,” which doesn’t quite get the sense of the point I was trying to convey.