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.

Clive Staples Lewis was a British writer, literary scholar, and Anglican lay theologian. He held academic positions in English literature at both Magdalen College, Oxford (1925–1954), and Magdalene College, Cambridge (1954–1963). He is best known as the author of The Chronicles of Narnia, but he is also noted for his other works of fiction, such as The Screwtape Letters and The Space Trilogy, and for his non-fiction Christian apologetics, including Mere Christianity, Miracles, and The Problem of Pain.