I suppose that the existence of something resembling a god is possible if we are actually living in a simulation. Even the christian god would be somewhere in that space of possibilities, though given the space of possibilities, that one specific possibility would still have to have extraordinarily low probability.

But let's say the christian god shows up on our world one day and says "hey all, yup, I'm totally real, now get on your knees and praise me or suffer eternal torment!"

I don't know about anybody else, but my atheism wouldn't so much as wobble. Why? Because I don't see atheism as disbelief in any specific entities. To me, it is the dismissal of the concept of divinity--that you can't have a fundamental authority or something fundamentally moral any more than you can have something that is fundamentally complicated--it is disbelief in the obligation to worship.