Which C.S. Lewis book presents ideas developed from his BBC radio talks?

Mere Christianity presents ideas developed from C.S. Lewis’s BBC radio talks.

The talks were broadcast during the Second World War, beginning in 1941, and were intended to explain basic Christian beliefs to a general audience. Lewis spoke in a direct, conversational style rather than using the specialized language of academic theology.

He later revised and collected the material in book form. Mere Christianity was published in Britain in 1952, combining material from the broadcast series with related talks and lectures. Its chapters discuss morality, belief, Christian doctrine, and the meaning of conversion.

The title does not refer to a separate denomination. Lewis used “mere Christianity” for what he regarded as the common core shared by historic Christian traditions. The work became one of his most widely read apologetic books and helped establish his public reputation as a Christian writer.