I've thought about this considerably in the past... I've come to the conclusion, when we're dead, we're dead, and that's it. It's depressing and hard to comprehend, but the chances are, it's true. Man created God, Heaven etc, because he fears death. The notion of there being an after-life after our real existence is just a mass dillusion brought on by fear. Sure Science hasn't been able to disprove the validity of Religion, but there is no hard evidence of there being another existence after this one... Science just makes more logical sense to me then "Faith".
You could deeply argue this topic for a long time... eventually you would end up with how it all started and how or what caused everything to be where it is today. Humans can't even begin to process that type of thinking... I know I can't. This universe came from something? What? A "Big Bang" a "God"? Well what created God? Or What caused the "Big Bang"? Like I said, your head will explode before you can come to a conclusion... All you religious people, enjoy your faith I guess, when you die it won't matter either way. That's just my opinion.