[posterchild]

This is what I think.
The idea of God was created by people because they need a reason to continue being good-hearted. People get tired of being "sin-free" when they are watching the hedonists soak up the self-gratification that being a sinner provides.
They start wondering why the hell they follow the straight and narrow when nobody else is.

This is where God comes in...

The notion of an eternity of jubilation in exchange for a life-span of purity is the incentive to remain unsoiled amongst all of the filthy filthy souls destined for fire and brimstone.

Not only that, but God serves as a wonderful answer to any question that can't be answered indefinately. Who created me? Who decides my destiny? Who put the stars in the sky?
Duh, if you can't figure it out it must have been God.

This, of course, is only my opinion...