I think this is a key point which causes a lot of difficulty where it need not if we can come to a simple and reasonable acceptance of the normal meaning of the words used.
Non physical things have no physical evidence and therefor cannot be said to exist.
The nature of an existent thing qua thing is that it has physical extension and therefor there is physical evidence of its existence. No physical evidence = no physical existence. So gos is not a thing in and of itself.
Now we generally admit a secondary form of existence, that of identifiable patterns or distinct organizations of things. For example, the pattern we call the Mona Lisa is recognizable as a distinct entity independent of the substrate which forms the pattern, be it paint or elections or ink.
In this case the concepts of god, while vague and varied from group to group, are distinguishable as a particular set of concepts. I don't think an atheist would take exception to god existing as a concept which finds instances in the various believers and writings.
Now the actual point of contention is whether or not this concept of god has any extension outside the beliefs of those who form it.
When a believer says there god is "real" that is the essence of the claim they are making. They are saying that in spite of not having any actual physical extension or existence, their god has some extraordinary existence outside their concept of it.
It is hardly a remarkable request on the part of the atheist to be shown proof of this existence more substantial that "I just believe" or "its in my book of Jewish myths."
Mere concepts are a dime a dozen and human history is littered with myths of gods which turned out just to be stories. If you want to convince a rational person or lay claim to being rational yourself, you need proof that your concept of god points to an actual god.
Anything less is a waste of time no matter how you feel or believe.
