Which 1952 book by C.S. Lewis developed from his BBC talks on Christianity?

C.S. Lewis’s 1952 book developed from his BBC talks on Christianity is Mere Christianity.

Lewis delivered a series of radio talks during the Second World War, beginning in 1941. The broadcasts were aimed at a general audience rather than at specialists, and they explained shared Christian ideas in accessible language. Lewis later revised and combined the material into the book Mere Christianity.

The title refers to the central beliefs shared by the major Christian traditions, not to a separate denomination. The book is often confused with Lewis’s other apologetic works, including The Problem of Pain and Miracles. Its structure grew from broadcast talks, which is why its prose often retains a direct, spoken quality.