Which C.S. Lewis book was based on his BBC radio talks?

Mere Christianity was based on C.S. Lewis’s BBC radio talks.

Lewis delivered the talks during the Second World War, beginning in 1941. They were intended to explain and defend core Christian beliefs to a broad public audience, not to promote one particular Christian denomination.

The broadcasts were later revised and collected in book form. Mere Christianity was published in 1952, bringing together material from the talks and from earlier printed versions. Its title refers to the shared beliefs that Lewis saw as common to historic Christianity.

The book is often confused with The Problem of Pain, which was published in 1940 and was not based on the BBC broadcasts. Lewis also wrote the later radio-related book Beyond Personality, but Mere Christianity is the work most directly associated with the wartime talks.