Which Christian book by C.S. Lewis developed from his BBC radio talks?

Mere Christianity developed from C.S. Lewis’s BBC radio talks.

Lewis delivered the talks during the Second World War, beginning in 1941. The broadcasts addressed basic Christian beliefs in accessible language for a general audience, and their popularity led to printed collections. The material was later revised and published in book form as Mere Christianity.

The book is organized around common Christian beliefs rather than the distinctive doctrines of one denomination. Its famous discussions include moral law, human wrongdoing, Christian virtue, and the person of Jesus. Lewis did not present it as a complete systematic theology; instead, he aimed to explain what he regarded as the shared core of historic Christianity.

The title refers to “mere” Christianity in the older sense of basic or common Christianity, not to something trivial. The book’s radio origins help explain its conversational style and clear structure. It became one of Lewis’s most widely read nonfiction works.