The BBC first aired the radio talks that became C.S. Lewis's book Mere Christianity. Lewis delivered the broadcasts during the Second World War, beginning in 1941.
The talks were designed for a broad British audience rather than for academic theologians. Lewis discussed moral law, human behavior, Christian belief, and the central doctrines shared by major Christian traditions. His direct, conversational style helped make difficult religious ideas accessible to radio listeners.
The broadcasts were later revised and collected in print. Mere Christianity was published as a book in 1952, with the title referring to the common core of belief Lewis believed was shared by historical Christianity. It is not a complete denominational theology and does not attempt to settle every difference among churches.
The wartime broadcasts helped establish Lewis as a public religious communicator. They also contributed to the popular association between his name and Christian apologetics, alongside works such as The Problem of Pain and The Screwtape Letters.