What is the main theme of the story?

From the very opening pages of the book, the theme of good against evil is prevalent. The story opens with the narrator struggling alone on the road to Ransom's, trying to decide if what he is facing is evil or good. Even when he decides what he is commissioned to do is good, he fights against doing it.

We see this theme throughout the story as Ransom battles the Un-Man, trying to keep Evil from overcoming the Green Lady. Evil overcomes Professor Weston, capturing his body for its own devices. Weston resists the evil force, but his pride causes him to be vulnerable and in turn be overcome.

Ransom ponders the desire to live the pleasures of life over and over, just as he wishes to drink the nectar of the beautiful bulbous growths given for his pleasure. He decides that maybe this desire to have pleasure over and over is the root of evil. Of course, he decides, the love of money is really the root of evil. But maybe the desire for money is the desire to have pleasure in life repeated over and over. Is this not the true evil, he muses?