Which C.S. Lewis book presents a radio-based introduction to Christian belief?

The C.S. Lewis book presenting a radio-based introduction to Christian belief is Mere Christianity. The book developed from talks Lewis gave for the BBC during the Second World War.

Lewis first broadcast the talks in several series between 1941 and 1944. He aimed to explain ideas shared by many Christian traditions rather than defend one denomination’s distinctive doctrines. The broadcasts were later revised and collected into book form.

Mere Christianity is organized into sections commonly titled Right and Wrong as a Clue to the Meaning of the Universe, What Christians Believe, Christian Behaviour, and Beyond Personality. Lewis discusses morality, belief, virtue, pride, forgiveness, and the Christian understanding of God.

The title refers to what Lewis regarded as the common core of Christianity, not to a separate church or denomination. Readers sometimes assume the book is a transcript of one speech; it is instead a revised collection drawn from multiple broadcast series.