God is not a superior power for everybody (or at least not an omnipotent being). I would say for Christians it's the creative power of the universe and the power who creates our own free will (if I'm wrong just correct me).
I rather like that idea. It reminds me of what John Fowles argued in The French Lieutenant's Woman
: "If God exists, it can only be the Freedom that allows other freedoms to exist." And I think it's close to the Christian point of view.
Free will to me has religious origins. If everything is material that means we would be predetermined by chemical reactions. I personnally can't believe in that. But that's just belief and I assume it.
Believing in some you can prove the existence is rather silly because if you can prove it you just CAN'T deny it.
If you believe in something or in something else, it just mean you can't prove it. But by the way science can only focus on material things. So what about abstraction? Sentimentality: love, hate, beauty, ugliness, etc. What has science to do with that?