"Mere Christianity" is a thought-provoking book written by C.S. Lewis, where he explores the fundamental principles of Christian faith. Through a series of insightful and logical discussions, Lewis presents a compelling case for the existence of a moral order and the rationality of Christian beliefs. He addresses topics like morality, human nature, and the essence of God's existence, making it a timeless work that continues to engage readers in profound contemplation about the foundations of Christianity. "Mere Christianity" has sold millions of copies worldwide in over half a century. It is one of the most popular introductions to the Christian faith ever written.