Which C.S. Lewis book is based on his radio broadcasts explaining basic Christian belief?

Mere Christianity is the C.S. Lewis book based on radio broadcasts explaining basic Christian belief. Lewis delivered the talks for the BBC during the Second World War, and they were later revised and published in book form in 1952.

The work is arranged in four parts: “Right and Wrong as a Clue to the Meaning of the Universe,” “What Christians Believe,” “Christian Behaviour,” and “Beyond Personality.” Lewis aimed to present ideas shared across historic Christian traditions rather than defend one denomination’s distinctive practices.

The title does not mean that the book is a complete history of Christianity or a guide to every church doctrine. It refers to what Lewis called “mere” Christianity: the common core of belief held by Catholic, Protestant, and Orthodox Christians. The book became one of his most widely read works of Christian apologetics.