I was going to go to sleep, but now I'm not quite sleepy enough to sleep. So let's see if this is coherent.
Lewis opens by reiterating that atheism is too simple. So is, he says, a version of Christianity he calls 'Christianity-and-water' - a view that says that there is a good God in Heaven and everything is all right. Ignoring all of the difficult doctrines. He calls both of these views 'boys' philosophies'.
He goes on to say that asking for a 'simple' religion is ridiculous. The world, after all, is not simple. So how could a religion be simple and have anything to do with the world we live in?
Don't most religions claim to be simple, at least at their core? Certainly the actual living out of the faith is never simple, but I think most of them view themselves as based on simple principles. Similar to the way the world and the universe, all of reality even, can be said to be based on simple principles, the laws of nature. Physics. 'Simple' can be a very relative term. That being said, I can see Lewis' point. If a religion or a philosophy is too simplistic it has no weight, no depth or flavor. It is useless in real life because it tends not to be flexible enough. Then again, overly detailed philosophies have the same problem.
Lewis also says that reality is odd. Which is why he finds Christianity believable. It mirrors reality. It is both complex and odd.
Well, yeah. But that doesn't mean that it's divinely inspired or anything. If reality is complex and odd, then wouldn't people, left to their own devices, project those same characteristics onto their religion and gods?
He then goes on to contrast Christianity's view of the world with Dualism. Christianity, according to Lewis, views the world as something good that has gone wrong but still retains a memory of its goodness. Dualism, on the other hand, says that there are two equal beings/forces. One Good, one Evil. They are both eternal and separate, depending on nothing for their existence, but forver opposing the other.
Lewis says that this is problematic because one can be good merely for the sake of goodness, but not bad for the sake of badness. People love good and will do things only because they are good. He does not believe that people ever do bad simply because it is bad. There are always other reasons, greed, pain, sorrow, anger, etc.
And in another bid to lose me, he says in the middle of this: 'The nearest we can get to it is in cruelty. But in real life people are cruel for two reasons - either because they are sadists, that is, because they have a sexual perversion which makes cruelty a cause of sensual pleasure to them' - I'll just be over here, hitting my head into my desk. I know, I know, time and context. Lewis did not have the understanding that we do now. It still gripes me.
Lewis' point is that the Evil force cannot be wholly independent of the Good force because the Evil force is predicated on the rejection of the Good. We're judging one better than the other because it adheres to a standard of goodness. But where did the universe get that standard of goodness for the Good force to adhere to? So there must be a third being, one who generated the other two. Otherwise, the Good generated the Evil as a contrast to itself and that still knocks the power balance out of whack.
He seems to be saying, almost, that Christianity is 'corrected' Dualism. Because it's put the Evil power into the proper context of having been a created being who chose to be Evil in opposition to the Good.