Which C.S. Lewis book was adapted from his 1940s radio broadcasts about Christianity?

Mere Christianity was adapted from C.S. Lewis’s 1940s radio broadcasts about Christianity.

Lewis delivered a series of talks for the BBC during the Second World War, beginning with Broadcast Talks in 1941. The talks were intended for a broad audience rather than only for theologians, and they explained shared Christian ideas in direct, accessible language.

The material was later revised and published in book form. The title refers to what Lewis saw as common ground among major Christian traditions, not to a new denomination. The book is arranged in sections that discuss moral law, Christian belief, and Christian conduct.

Lewis became an especially familiar public voice in wartime Britain because of these broadcasts. Mere Christianity should not be confused with The Problem of Pain, his earlier book-length discussion of suffering, or The Four Loves, which examines different forms of human affection. Its origin in radio talks is the key identifying fact.