Which C.S. Lewis book began as a series of BBC radio broadcasts?

Mere Christianity began as a series of BBC radio broadcasts by C.S. Lewis.

Lewis delivered the first broadcast talks during the Second World War, beginning in 1941. Further series followed in 1942 and 1944. The talks were intended to explain and defend basic Christian beliefs to a broad British audience during wartime.

The material was later revised and collected into book form. The title Mere Christianity refers to the common core of belief shared across many Christian traditions, rather than to a separate denomination. The book is divided into sections including “Right and Wrong as a Clue to the Meaning of the Universe” and “What Christians Believe.”

The broadcasts were not originally delivered under the final book structure. Lewis edited and reorganized the spoken material for publication, which is why the book is best understood as a developed collection of his broadcast arguments.