Which C.S. Lewis book presents radio talks later adapted from broadcasts during World War II?

Mere Christianity presents material C.S. Lewis originally delivered in radio broadcasts during World War II. The talks were broadcast by the BBC between 1941 and 1944 and later revised and collected as a book.

Lewis aimed to explain central Christian ideas in accessible language rather than defend one denomination. The book is organized around broad themes, including morality, Christian belief, and the implications of the life and teaching of Jesus. Its title refers to the shared core of Christianity.

The broadcasts made Lewis widely known beyond academic and literary circles. The book is sometimes described as a transcript, but Lewis revised the talks for publication, so the printed text is an adapted version. The work remains one of his most frequently read books of Christian apologetics.