Well if God and Satan did exist, Satan is supposed to be the lord of deceit and lies. So it stands to reason that he would spread lies to get as many people doing his work as possible and condemning themselves to Hell. Isn't it possible then, that Satan was responsible for writing the bible in a way that would divide mankind and make them argue over its meaning as much as possible? Perhaps he also came up with the whole concept of an afterlife in order to eliminate the only true good in the world- people being altruistic and kind with no hope of any reward for it?

Once you introduce the idea of a super-powerful being trying to lie to you as effectively as possible, how could you ever trust any information again considering that the most generally accepted facts would be the most likely to be tampered with?

It's a good job neither God nor Satan exists really, I think it would be horribly confusing to think about too much.