Like Mammuthus, I agree with Mark. I think there are three different positions involved -
I don't know whether there is a God
I do not believe in God
There is no God.
I adopt the second position - an absense of belief. And I call myself an Atheist, but some may quibble about this.
The first and third statements both imply that there is some way of telling whether God exists or not, and as I have not yet heard of one, both statements are meaningless to me.
Mike.