It is simple. If that being proves that is a god or God. Then basically under the evidencies it would be admitted. Now, what god would be?

And what god would be? the one described in the Old Testament, in the New testament, in the Coran, or a god that is not described in any other book. If that god came and said, I did not care a $#@! about you, and nothing that in your books was done by me, as I did not care of you. Just I passed here because I was bored. What would be the reaction of the believers?

But, as god is as real as Gandalf. This scenario won't happen ever. But if happened, as all atheists and agnostics ask for evidence of its existence. As many scientist mind, would reevaluate its hypothesis.

What would you do if was proven that there is no god, and they never existed? Would you reevaluate your believes. That is the point of fantaticism, that even if there is no evidence of their belief, and even evidence against it, they continue believing on it, dismissing it.