Its one of those things that can be debated endlessly, while never getting any closer to being solved. If it does exist then it is beyond our understanding anyway.

My own opinion on the matter is that God, the Devil etc. are all a creation of the human mind. My reasoning is that as a species we need order, otherwise we'll end up wiping each other out. As every person has free will, then without rules and consequences, people would be able to do anything they want (if they had no morals).

In other words, the promise of eternal life and bliss for being good, and of punishment, fire and brimstone if you're not, is the ultimate way of maintaining order of some kind.

Complicated and not very well thought through I know, but just my two cents.