Mere Christianity is a book by C. S. Lewis, adapted from a 1943 series of BBC radio chats broadcast while Lewis was an Oxford don during World War II. The transcripts of the broadcasts, expanded into book form, originally appeared in print as three separate pamphlets, The Case for Christianity, Christian Behavior, and Beyond Personality.

The title, Mere Christianity, indicates the intention of Lewis, an Anglican, to describe the Christian common-ground. He aims at avoiding controversies to explain those things that have defined Christianity in nearly all places and times. Lewis restates the fundamental teachings of the Christian religion, for the sake of those basically educated as well as the intellectuals of his generation, for whom the jargon of formal Christian theology did not retain its intended meaning.

Of course, the book has not been received entirely without controversy. For example, the chronicle of Lewis's conversion from atheism contains some of the author's reasons for believing which, as may be expected, some have found to be compelling while others have ridiculed. Further, both Christian and non-religious critics have suggested that Lewis created common ground out of beliefs and sentiments that can only be made to appear similar by being purposely vague. In fact, he makes explicit his use of purposeful vagueness at the beginning of the book, when he describes the common ground of all religions, with his point being that Christianity is not mathematics, even if like mathematicians, Christians may claim that there is only one right answer.

Mere Christianity is widely admired and influential across a spectrum of trinitarian Christians, which may attest to the author's success in accomplishing the aim of restating theology in a way that avoids many controversies. The work is also widely admired by many nontrinitarian Christians.

The title has influenced Touchstone: A Journal of Mere Christianity and William Dembski's book Mere Creation.

See also: The Inklings