Which C.S. Lewis book adapts his 1940s BBC broadcasts on Christianity?

Mere Christianity adapts C.S. Lewis's 1940s BBC broadcasts on Christianity. The book developed from three broadcast series delivered during and shortly after the Second World War: The Case for Christianity, Christian Behaviour, and Beyond Personality.

Lewis aimed to explain central Christian beliefs in accessible language rather than defend one denomination. The resulting work discusses morality, human nature, repentance, Christian conduct, and the doctrine of the Trinity. Its title refers to the shared core of Christianity, not to a separate church or denomination.

The book was assembled in stages. Broadcast material first appeared in print as separate works, including Broadcast Talks and Christian Behaviour, before being combined in the form commonly known as Mere Christianity. It became one of Lewis's most influential apologetic works and is often read alongside The Problem of Pain and Miracles.