Which C.S. Lewis book adapts his BBC wartime talks on Christianity?

Mere Christianity adapts C.S. Lewis's BBC wartime talks on Christianity.

Lewis delivered a series of radio broadcasts for the BBC between 1941 and 1944, during the Second World War. The talks addressed basic Christian ideas in language intended for a broad audience rather than for specialists. Their popularity led to revisions and publication in book form.

The resulting volume, Mere Christianity, was assembled from three earlier works: Broadcast Talks, Christian Behaviour, and Beyond Personality. It presents Lewis's case for shared Christian beliefs, including moral law, repentance, and the Christian understanding of God and Jesus.

The title does not refer to a separate denomination called “mere Christianity.” Lewis used the phrase for what he regarded as common ground among the major Christian traditions. The book is therefore a later literary form of broadcast material, not a verbatim transcript of one single speech.