Yesterday I tweeted Humans don't need God to exist but God needs humans to exist, once we stop believing he will disappear this was retweeted a few times and even compared to Terry Pratchett (I'm flattered, I must read his books now!) so I thought I would elaborate.
I can't help but think that nearly everything humans used to attribute to Gods in regards to our own existence has been explained by science. For example in his bookOrigin of the Species Charles Darwin was able to explain brilliantly how we evolved as a species and how we originated, the reason his work was so controversial is because he took away the need for God to explain our origins. There is also the big bang theory, which takes away the need for God when it comes to explaining the origins of our planet, solar system and universe. Of course, these theories don't disprove God, some theists argue that all this was part of God's plan, if you're a deist you would say God started this off then took a backseat. I agree with Stephen Hawkings when he says that God was not needed for any of these things to happen.
My theory about God is that he exists in the imagination of people, that's why there are so many interpretations of God and over 3,000 Gods that have been worshipped throughout history. This is why I believe that if people simply stopped believing in God he will die because humans are his lifeline, as for humans we will carry on living.