I've had some similar idea to this. What makes God--if I'm wrong and he exists--any different than an alien that is infinitely more advanced than we are? If God popped in one day and said, "What up?" how would you know it was him? A sufficiently advanced race might be able to perform any and every miracle we could think to ask from God as proof. So how would you know it was God and not some alien?
Even if God stepped in and stopped the aliens from impersonaing him, how would we know it wasn't an even more powerful bunch of aliens that intervened?
IsaacJ