God - Devil

I'm not trying to push anyone to atheism, I am simply explaining my logic. I respect other people's beliefs even if I myself think they are wrong, I would never put anyone down for them.

If some joker who lived in a cave for 28 years wrote a hella long book about a magical creature with unlimited power and knowledge today, we'd call him crazy and lock him up.
So, why believe what he did back then?
Or, say if someone claimed to of turned water red, parted an ocean, or called a bunch of bugs down to kill people, what would you think of them?
What if I were to tell you something along the lines of:
I have foretold that a evil monster with 20, 000 arms, 10 eyes, and 1 leg will come to Earth and give us advanced medical techniques that can heal any and all illnesses in the universe... BUT only if you pray to him once a day for the rest of your life and have faith in him.
You would not be able to DISPROVE that theory would you?
What would you do?