The more i think about it, i'm not sure it's possible for a "god" to exist, at least how the term is typically bandied about. We tend to think of "god" as being a being of undefinable power, but once you know something is real, it becomes definable. Once it's definable, does it still qualify as being a god?
It's like the "any sufficiently advanced technology is indistinguishable from magic," quote. What's the difference between "god," and "very technologically advanced alien?"
The real question is, once you are confronted with the unequivocal existence of this really advanced alien, what do you do next? Sure, you believe, you'd pretty much have to, but would one automatically worship? Somehow, i doubt it.
Maybe that's why the "ancient astronauts" haven't returned. I can hear it now: "These people are too weird. Let's not come back until they stop being so superstitious."