Okay, well X exists ok? And we don't know what it is. What is it? Well it's X. X is X. But that gets us nowhere at all, it's circular, of course. So to find out what "X" is we have to find out what it is not. But that makes no sense either because we can't find out what it is not until we find out something about what it is.
Substitute "X" for "God" and you see the problem.
Substitute "X" for "God" and you see the problem.