Lewis, one of the foremost Christian writers of the last century, took a chapter from The Golden Ass of Apuleius as his source for a fantasy. It became arguably the best of his novels; Lewis himself considered it his best and was disappointed by its failure with the public.
There is no specifically Christian content in the book. It is set in the tiny kingdom of Glome, "a little barbarous state on the borders of the Hellenistic world" in Lewis's description, in perhaps the third century BC. The pagan gods are treated as real, and rather primitive and terrifying.
Warning: Wikipedia contains spoilers.
The story is a retelling of the Greek myth of Cupid and Psyche, from the point of view of Orual, Psyche's jealous ugly sister (as she is seen in the usual telling). It begins as the memoirs of an old woman, bitter at the pain and injustice of the world. Far from resenting her beautiful sister, she loved Psyche and was devastated by her loss. Now, after a long and successful reign as queen of Glome, she looks back, not understanding what was wrong with her love for Psyche: it was too fierce and too narrow and without faith, and that had brought ruin.
In the shorter second section of the book we see that, having written the memoir, she considered it time to end her miserable life. Like Psyche, she did not die; and she went through a series of experiences that paralleled those of Psyche. In the end, by a way even harder than that of Psyche, she reaches understanding and acceptance.