The symbolism of the Christian Trinity is a running parallel throughout the novel, though because the action has been displaced to Mars where the people have not "fallen" the protagonist, Ransom, is forced to try to find the Trinity exisiting as a whole rather in the separate ways we here on earth must experience God.
A more tangible symbol of evil is the water serpent the Martians call "hneraki." Its existence puzzles Ransom, for if the Martians have not sinned, he wants to know why evil exists in the world. Ransom is told that "the forest would (not) be so bright, nor the water so warm, nor love so sweet, if there were no danger in the lakes."
Charles Moorman argues that Lewis "is proposing here a perfectly orthodox answer to the problem of the existence of evil: God allows evil in the universe for reasons which man does not understand but are eventually for man's own good."