Which C.S. Lewis book presents Christianity through four radio talks first broadcast in 1941?

Mere Christianity presents Christianity through four radio-talk series first broadcast in 1941.

C.S. Lewis developed the material for the BBC during the Second World War. The broadcasts were intended for a broad audience and focused on ideas shared across major Christian traditions, including moral law, human nature, and the central claims of Christian belief. Lewis’s conversational style helped make complex theological arguments accessible to listeners.

The material was later revised and published in book form. Mere Christianity is not a memoir, unlike Surprised by Joy, and it is not primarily a treatment of suffering, like The Problem of Pain. The title refers to the common or basic core of Christian belief rather than to a separate denomination founded by Lewis.