This one is purely out of curiosity. I'm not here to criticize your beliefs, or even to try and convert you. (Church is this way, boys!) It is on the same idea that many cultures develop the same mythology. (There's almost universally a bigfoot of some kind, for example) 'If they don't exist, why so many cultures know about them?'
The answer to this seems simple: They are based on real creatures. This could mean bigfoot is real, or this could mean that many people misinterpreted local primates of some kind.
It doesn't apply so well in the case of God. If there is no God, how have so many cultures come to recognize one or multiple supreme beings? Almost always with the same basic set of rules. 'Don't kill people', for example.