Which C.S. Lewis book presents radio talks later revised into a popular Christian apologetics work?

Mere Christianity presents C.S. Lewis’s radio talks, later revised into a popular Christian apologetics work. The book was assembled from broadcasts Lewis gave during the Second World War.

The original talks were broadcast by the BBC between 1941 and 1944. Lewis aimed to explain shared Christian beliefs in accessible language rather than defend one particular denomination. The final book combines material from Broadcast Talks, Christian Behaviour, and Beyond Personality.

The title refers to what Lewis called “mere Christianity”: the common ground of historic Christian belief. It does not mean a vague or minimal personal spirituality. The book is especially known for arguments about morality, human nature, and the identity of Jesus.