Mere Christianity grew from C.S. Lewis's BBC radio talks broadcast during World War II.
The talks were delivered between 1941 and 1944, when Lewis addressed a broad British audience about Christian belief. He aimed to explain shared Christian ideas in accessible language rather than defend one denomination's distinctive doctrines. The broadcasts were later revised and collected in book form, with the title Mere Christianity.
The book is often remembered for its discussion of morality, reason, and the famous trilemma concerning Jesus. It is not a novel and does not present a complete history of Christianity. The radio origin also explains its conversational tone: Lewis was speaking to listeners who might have had little formal theological training.