If there is no God would it be necessary to invent him?

Free will is the dynamic at the core of reincarnation. If you don't want to reincarnate, you won't. At the physical and non-physical level - like matter and anti-matter - you represent a tiny fragment of the energy that formed the Cosmos, the energy that always was and always will be. You're eternal, as is your consciousness. You could, however, choose to shut down and blink out, rather than shine on - if that's what you want to do. As for hell, that's an invention born of man's fear of divine wrath. Free will lets us create our own hells.