Which C.S. Lewis book explains Christianity through a fictional conversation with a demon?

The Screwtape Letters explains Christian ideas through a fictional exchange between demons. C.S. Lewis published the book in 1942, presenting a series of letters from the senior demon Screwtape to his inexperienced nephew Wormwood.

Screwtape advises Wormwood on how to tempt a human patient away from God. Lewis reverses the usual perspective: readers hear demonic strategy rather than direct instruction from a virtuous narrator. The technique allows him to satirize spiritual complacency, distraction, pride, and self-deception.

The work began as letters published in the Anglican newspaper The Guardian before appearing in book form. It is not a conventional novel with a continuous plot, but its correspondence creates a sustained fictional narrative and a framework for theological and moral commentary.