Well, Jim, people decided there is one (a God)...then they give it a name. That's how it works. It's like when Walt Disney came up with a talking duck...and he named it "Donald". You have to come up with the character first, then the name. As soon as you have a character, it needs a name so you can talk to other people about it and they know what you're referring to.

In fact, everything needs a name...otherwise we have no way of talking about it. What would we call "global warming" or "anchovies" if the words didn't exist yet?

Now as to whether there IS a supreme being or not...I don't exactly see that you or I are in a position to prove that one way or another. ;-)

But even if such a being was mythical (like Donald Duck), it would still require a name, a word that specifically designates it. Otherwise we couldn't discuss it very readily, could we?

You might just as well ask how we can call Santa Claus anything at all! ;-) It makes no difference whether or not God really exists as to whether we can call God by the name "God".