What was the title of C.S. Lewis's 1943 book defending Christian belief through radio talks?

Mere Christianity is the title of C.S. Lewis’s 1943 book defending Christian belief through ideas first presented in radio talks. The broadcasts were delivered for the BBC during the Second World War and later revised into book form.

Lewis designed the talks for a broad audience rather than for specialists in theology. He began with moral experience and the idea of a shared moral law, then discussed Christian doctrines and the practical implications of faith. The book is divided into sections commonly titled “Right and Wrong as a Clue to the Meaning of the Universe,” “What Christians Believe,” “Christian Behaviour,” and “Beyond Personality.”

The title refers to what Lewis called “mere Christianity”: core beliefs shared across historic Christian traditions. It does not mean Christianity is unimportant or minimal in every sense, and Lewis did not intend the phrase to erase all denominational differences.

The book became one of Lewis’s most influential works of apologetics. Its radio origins also helped make Lewis’s voice and arguments familiar to a large British wartime audience.