Which C.S. Lewis book developed his wartime radio talks into a Christian apologetics work?

Mere Christianity developed C.S. Lewis’s wartime radio talks into a Christian apologetics work.

The BBC broadcast Lewis’s talks during the Second World War, beginning in 1941. He later revised and combined the material for publication, first as separate broadcast-related volumes and eventually as Mere Christianity in 1952. The book presents arguments and reflections on morality, Christian belief, and the distinctions among major Christian traditions.

Lewis deliberately aimed to discuss what he regarded as shared Christian convictions rather than defend one denomination’s full system. That is why the title uses “mere” in the older sense of basic or essential, not “unimportant.” The work became one of his most influential nonfiction books.

Readers sometimes confuse it with The Problem of Pain, another Lewis apologetics book, but that earlier work was not based on the BBC talks. Mere Christianity grew directly out of the radio broadcasts.