To be fair, if you asked God for a sign and he gave it to you, then there would be no need to have the sort of faith prescribed by most major world religions. You would simply know.
Depending on your own interpretations, receiving evidence of God's existence (outside of an "end times" scenario) would immediately disprove most religions. They say you need to have faith that God is there and taking care of you and hearing your prayers - now you no longer need to have faith, so whichever sort of God was revealed to you, it's not the one from the bible.
What does the bible have to do with believing in God though?
I think that book is where most people who don't believe have this grudging hangup.