Which C.S. Lewis book presents Christianity through radio talks later revised into a single volume?

Mere Christianity presents Christianity through radio talks that C.S. Lewis later revised into a single volume. The broadcasts were delivered for the BBC during the Second World War, beginning in 1941, and the book was published in 1952.

The original broadcasts were arranged in three series: “The Case for Christianity,” “Christian Behaviour,” and “Beyond Personality.” Lewis spoke directly to a broad public audience rather than assuming specialist theological knowledge. He later edited and expanded the material, and the resulting book became one of his most influential works of Christian apologetics.

The title refers to what Lewis viewed as the central beliefs shared by the major Christian traditions. It does not present a complete denominational system or a detailed history of Christianity. Instead, it develops arguments about morality, human behavior, belief, and Christian doctrine.

Mere Christianity is often confused with The Problem of Pain because both are major apologetic works by Lewis. The radio-broadcast origin, however, specifically identifies Mere Christianity.