C.S. Lewis delivered the talks later adapted into Mere Christianity on BBC radio. The broadcasts began during the Second World War, with the first series airing in 1942 and later series following in 1943 and 1944.
Lewis was invited to explain basic Christian ideas to a broad audience rather than address specialists. His calm, reasoned broadcast style helped make him widely known beyond university and literary circles. The talks were later revised and published in book form.
Mere Christianity is therefore not a single speech or a novel. It is a book assembled from several radio-talk series, with revisions and additional editorial shaping. The title refers to the common core Lewis believed was shared by the major historic Christian traditions, rather than to one denomination.