C.S. Lewis's Mere Christianity began as broadcasts for the BBC during World War II.
The original talks were delivered over BBC radio between 1941 and 1944. Lewis was invited to explain and defend basic Christian beliefs in language accessible to a broad listening audience, including people serving in wartime Britain.
The talks were later revised and collected into book form. Mere Christianity is organized around common moral experience, Christian belief, and the distinctions among Christian virtues and doctrines. It was not written as a denominational statement for one church.
Lewis became a familiar wartime broadcaster, but the broadcasts were not live sermons in a church. Their radio origin explains the direct, conversational style of the eventual book and its emphasis on ideas that Lewis believed were shared across historic Christianity.