Which C.S. Lewis book grew out of his wartime radio talks broadcast from 1941 to 1944?

Mere Christianity grew out of C.S. Lewis's wartime radio talks broadcast from 1941 to 1944. The talks were later revised and collected into a book, first published in 1952.

Lewis was invited to speak for the BBC during the Second World War. His broadcasts presented a broad defense of Christian belief rather than an argument for one particular denomination. The resulting book is arranged in sections commonly known as Right and Wrong as a Clue to the Meaning of the Universe, What Christians Believe, and Christian Behaviour.

The title is often mistaken for the name of the original radio series, but the broadcasts had different titles, including The Case for Christianity and Christian Behaviour. Mere Christianity is the later book title. The work became one of Lewis's most widely read examples of twentieth-century Christian apologetics.