Mere Christianity is a Christian apologetical book by the British author C. S. Lewis. It was adapted from a series of BBC radio talks made between 1941 and 1944, originally published as three separate volumes: Broadcast Talks (1942), Christian Behaviour (1943), and Beyond Personality (1944). The book consists of four parts: the first presents Lewis's arguments for the existence of God; the second contains his defence of Christian theology, including his notable "Liar, lunatic, or Lord" trilemma; the third has him exploring Christian ethics, among which are cardinal and theological virtues; in the final, he writes on the Christian conception of God.
Mere Christianity was published in the United Kingdom by Geoffrey Bles on 7 July 1952. While initial reviews to the book were generally positive, modern reviewers were more critical of it, and its overall reception was relatively mixed. The praise was primarily directed to Lewis's humorous, straightforward style of writing; the criticism was primarily around the validity of his trilemma, which defends the Christian doctrine of the divinity of Jesus, and how he should have considered providing more choices.