The BBC radio series that helped make C.S. Lewis widely known during World War II was Broadcast Talks. Lewis delivered the talks in Britain between 1941 and 1944, presenting Christian ideas in direct language for a broad listening audience.
The broadcasts were later published as a book titled Broadcast Talks in 1942. Subsequent collections included Christian Behaviour and Beyond Personality. These works were eventually revised and combined into Mere Christianity, published in 1952.
The titles can cause confusion because “The Case for Christianity” is another related collection of Lewis’s wartime broadcast material, while “Christian Behaviour” was a later book section based on additional talks. Broadcast Talks is the specific early collection associated with the first series and with Lewis’s rise to national prominence as a Christian speaker.