as you say, this battle is going on in your mind. the important thing is that you learn to try to separate what is going on in your own head and what is happening in reality, this is the crux of your condition. whilst many believe in god and whatnot, few really see this imaginery battle betwen god and the devil as actuality, more a metaphor for life i think. thinking about the good and evil ding dong will just enhance stress. it is counter productive. if you are a believer as you say, then perhaps focus on the good that exists in/out of religion rather than the bad.
i think you really be ought to talking to a specialist professional, or a support help group. if your antipsychotic drugs are not woring then you should discuss this with your gp. plenty of proper help available if you really are looking to understand your self more or get better.
http://www.rethink.org/