Which C.S. Lewis book adapts his BBC radio talks into a basic explanation of Christianity?

Mere Christianity adapts C.S. Lewis’s BBC radio talks into a basic explanation of Christianity.

The talks were broadcast during World War II, beginning in 1941, when Lewis was already known for his clear public speaking. He first delivered the material in three broadcast series: Broadcast Talks, Christian Behaviour, and Beyond Personality. The talks were later revised and collected in book form as Mere Christianity.

The title refers to the shared core of Christian belief rather than to one denomination. Lewis organized the work around moral reasoning, Christian doctrine, and the implications of belief. Because it grew from radio broadcasts, the prose is direct and conversational, helping explain why the book remains one of his most widely read works.