What was the title of C.S. Lewis’s 1943 book of Christian apologetics based on radio talks?

C.S. Lewis’s 1943 book of Christian apologetics based on radio talks was Mere Christianity.

The work grew from broadcasts Lewis delivered for the BBC during the Second World War. Those talks addressed shared Christian beliefs and were intended for a broad audience, including listeners who were uncertain about religion. Lewis later revised and expanded the material into book form, arranging it in sections on morality, Christian belief, and Christian behavior.

The title refers to what Lewis called the common or central beliefs held across many Christian traditions. It does not present a complete guide to one denomination’s practices. The broadcast talks were first collected in separate volumes, including Broadcast Talks and Christian Behaviour, before being brought together under the title Mere Christianity. The book became one of Lewis’s most widely read works of religious argument.