The way I see it, one side can never "really" lose. You destroy one part of it and another part takes it's place. Then while in the process of destroying that part, one or more new parts are created. "Part," of course, can be replaced with whatever you desire -- just as light and dark could be replaced with good and evil.
You can't have one without the other though. If everything was "light," for instance, there not only wouldn't be darkness, but no "true" light either. It would just be a whole lot of undefinable (don't think that's really a word...), meaningless nothing. The same would go for all darkness too, I suppose, even though darkness is the absence of light to begin with.
... That's just my view of it though. In reality, I think it might simply come down to not being able to come up with a rational way of explaining how one can exist without the other.
As far as religion goes, I personally have no religion, nor have I held a Bible, let alone read one. I do believe that, given enough time, anything can be compared to anything else. In the end, no matter how "original" a story or idea is, it has many parts stemming from one or more basic concepts. Meaning if you remove the details and simplify the remainder as far as possible, you're bound to be able to compare multiple stories and call them similar or exactly the same. If you look at Kingdom Hearts and the Bible as wholes though, I don't think you'd be able to compare all that much.
