Jumping forward into Duncan's second post on the debate, he summarizes the ideas of objective and subjective existence nicely in the following two paragraphs.
There are two implications raised here. One - that a proposition considered to "objectively exist" does, in fact, objectively exist if it can be discovered (or rediscovered) whether or not humans have knowledge of it, or have written of it. For instance, there is gravity on Mars as well as in the Eta Carinae star system. We can say gravity exists because it is consistent in all contexts that we laypeople care about. Two - and more relevant to me today - is that the concept of objective existence leads us to the logical conclusion that "gods" do not exist. In other words, the concepts of Yahweh, Allah, Zeus, Odin, Krishna, Horus would not exist here, or across the universe, without humans to supply them. An alien life form, if it exists, might have a concept of a higher power, but it would be nothing like the stories we have been telling ourselves for thousands of years.
This “real” reality is what we could and should call material reality—not in the sense that it’s made of atoms (because atoms are real, and what are they made of, eh?), but in the sense that it exists in and of itself, as the necessary being. Fictional realities are not material realities, because their whole existence is contingent on the subjective perceptions and thoughts of some observer who is thinking about them. Santa Claus, for example, “exists” only in a fictional “reality” whose existence depends on people thinking about him and telling kids about him and so on. If we could wave a magic wand that would make everyone forget about him, and make every reference disappear from our art and literature and so on, then his “existence” would likewise cease, because it’s contingent on the perceptions of at least one observer.
Gravity, on the other hand, is an aspect of material reality, even though it’s not made of atoms. Wave the same magic wand, make people forget about gravity, delete every reference to gravity from literature and art, even render every sentient being in the universe unconscious so they can’t think about anything, and planets will still orbit their stars and rain will still fall down. Material reality does not depend on the perceptions of any third party, it exists in and of itself.
I think that, as much as any thought experiment you can perform about a god, is sufficient to demonstrate the absurdity of the claim that a personal god exists.