[QUOTE=BrainGlutton]
Actually, I have my doubts as to whether even God could prove the existence of God (except by reaching directly into your mind and tweaking your belief-system, but that would be brainwashing, not proof).
[/QUOTE]
This is, on one hand, a great point–but on the other hand, at some point a demonstration of omnipotence is so convincing that a tricksy alien may as well be a God.
For example, for an omnipotent God, it would be trivial to make the stars tonight spell out, “Lay down your weapons, for I am God!” It woudl be trivial to make the message instead be the most inspiring, beautiful, and persuasive bit of theological poetry ever written. It would be trivial to have the message appear to each human in the primary language they can read, and for those who cannot read, it would be trivial to allow them to hear the poem spoken by the stars. It would be trivial for an omniscient God to think of an even more persuasive form of proof of its existence, if its desire was to provide every human being with adequate proof. Such a God would know what would convince each one of us and could, using special omnipotence powers, provide us with that proof.
If an alien civilization could do the same, then that alien civilization may as well be God for all practical purposes.
The only problem with such an event would be the question of insanity. How would you distinguish between the perception of such an event and the total retreat into a fantasy world? As long as my secondary experiences (talking to others, looking at photographs of the event, etc.) were consistent, I’d probably conclude that it had actually happened.
Again, though, proving Its existence is the easy part. If an omniscient, omnipotent, omnibenevolent God wants me to believe that omnibenevolent bit, it’s going to have some hard questions to answer. Being omnipotent, of course, they won’t be that hard :).
Daniel