Which C.S. Lewis book explains Christian belief through fictional conversations with a demon?

The C.S. Lewis book that explains Christian belief through fictional conversations with a demon is The Screwtape Letters.

Published in book form in 1942, the work consists of letters from Screwtape, an experienced demon, to his inexperienced nephew Wormwood. Screwtape gives advice about tempting a human being, while Lewis uses the reversed viewpoint to examine pride, distraction, prayer, pleasure, and moral choice.

The idea began as a series of letters published in the Anglican newspaper The Guardian. Lewis later gathered them into a book. The demonic narrator is a literary device, not an endorsement of Screwtape’s advice; the satire works because readers are invited to recognize the weaknesses being described.

The book is often grouped with Lewis’s Christian apologetics, but it differs from a direct theological treatise. Its setting is fictional and its arguments are indirect. A concluding piece, “Screwtape Proposes a Toast,” was published separately in 1959 before appearing with later editions.