What was the title of C.S. Lewis's 1950 Christian apologetics book about faith and reason?

Mere Christianity is C.S. Lewis's 1950 book about Christian faith and reason.

The book developed from radio talks Lewis gave for the BBC during the Second World War. Those broadcasts were later revised and published in separate volumes before being collected as Mere Christianity. Lewis aimed to explain central Christian beliefs shared across many denominations rather than defend one church tradition.

The title refers to what Lewis presented as the basic or common core of Christianity. The book discusses morality, belief, Christian virtues, and the doctrine of the Trinity in language intended for a general audience.

It is often grouped with Lewis’s other apologetic works, including The Problem of Pain and Miracles. Unlike a novel, it is a sustained nonfiction argument designed to introduce readers to Christian ideas.