Which C.S. Lewis book was adapted from his 1940s radio talks on Christianity?

Mere Christianity was adapted from C.S. Lewis’s 1940s radio talks on Christianity.

Lewis delivered the talks for the BBC during the Second World War, beginning in 1941. The broadcasts were intended to explain core Christian beliefs to a broad audience rather than to promote a particular denomination. Their accessible style made Lewis a widely recognized religious broadcaster.

The material was later revised and collected in book form. The title Mere Christianity refers to the shared beliefs Lewis saw across historic Christian traditions. The book is divided into sections commonly known as Right and Wrong as a Clue to the Meaning of the Universe, What Christians Believe, Christian Behaviour, and Beyond Personality.