If a guy appeared before you and started doing magical things claiming to be god, would you be convinced? What if he had encyclopedic knowledge of everything, like, everything?
I ask this because if I hopped into a time machine and traveled one or two thousand years into the past with modern technology and gadgets and some way to access any data we have on the time period (who is going to be king in however many years, when a disaster is going to occur etc), would I not be indiscernible from god?
So, if someone appeared before us with magic and knowledge, would you be finally convinced, or remain skeptical, open to the possibility that this person simply has technology more advanced then we do?
(don't get too hung up on the time travel thing, it was just an example)
Overall, I doubt I would ever be convinced that said person was truly a God. I would, of course, be convinced they were powerful, if they did very obviously "impossible" things directly in front of me (changing form, conjuring things, healing wounds, etc.) with no sleight of hand, but still, no God.
However, if said guy started talking with the Voice of Legion and getting all glowy-eyed, like Aang in Avatar: The Last Airbender, moving all the elements and such, then I might bow down, in hopes that he'd share his power without requiring too much commitment! Hey, I can dream, can't I?
If someone appeared before me with provable (testable) magic and knowlege, I might be convinced they had knowledge and could perform magic. So what? This doesn't mean I would worship them or concider them a god.
In order for me to believe, they would have to regenerate a new limb on an amputee friend of mine, right before my eyes. Again, I still probrably wouldn't worship them.
If someone were truly a God, they could simply make me convinced. If they couldn't do that, they aren't really an omnipotent God, are they? A lesser God, something like Roman or Norse mythology, I'd be skeptical. And if they were carrying a big hammer that was breaking things and shooting lightning and thunder and told me to call them a God, I have enough of an interest in self-preservation to do so no matter what I believed ;)
If you were to time-travel to the past, it is highly unlikely that you would be able to speak the language coherently. Even if you only went to Dark Ages Europe you would not be able to make yourself understood.
And if the sort of character appeared to me, no, I am far to skeptical. It would take a lot more proof for me to even suspect that this person were a god. And then, it would take a lot for it to convince me that I should care. So it's a god. So what?
If it wants me to be subservient, it is my enemy.
if it wants me to be worshipful, it is arrogant.
If it wants to give me something, well, beware of Greeks bearing gifts . . .
When I saw this topic, what immediately popped into my head was when Christians ask "OK so what would be sufficient evidence for you to believe in God?? NOTHING would be acceptable to you!!! Because you are closed-minded!"
I know this isn't exactly what you were asking, but I've seen this question asked many times and I've never seen this simple answer from atheists:
EXACTLY the same thing(s) it would take for YOU to believe in Zeus! (Don't say Santa or the tooth fairy or anything, as that is more likely to trigger an emotional response since you are just trying to make them look foolish).