I hope that no reader will suppose that 'mere' Christianity is here put forward as an alternative to the creeds of the existing communions.... It is more like a hall out of which doors open into several rooms. If I can bring anyone into that hall I shall have done what I attempted. But it is in the rooms, not in the hall, that there are fires and chairs and meals. The hall is a place to wait in, a place from which to try the various doors, not a place to live in.
You must keep on praying for light: and, of course, even in the hall, you must begin trying to obey the rules which are common to the whole house. And above all you must be asking which door is the true one; not which pleases you best by its paint and panelling.
C.S. Lewis presented what would later become Mere Christianity as a series of radio talks given on the BBC from 1942-1944. The "mere" Christianity of C.S. Lewis is not a philosophy or even a theology that may be considered, argued, and put away in a book on a shelf. It is a way of life, one that challenges us always to remember, as Lewis once stated, that "there are no ordinary people," and that "it is immortals whom we joke with, work with, marry, snub, and exploit."
Mere Christianity is really 4 books in one - The first 'book' discusses basic morality, and the 'laws' of the Christian faith; basic doctrines and beliefs that all Christians share. The second is about the Christian ethic, how we live out our beliefs, and how they affect us. The third book is on behavior, or the psychology of our beliefs, and the effects of sin upon humanity. The fourth book tackles deeper doctrines and the cost of Christianity.
I want to share a few snippets from this magnificent book with you, my readers. And, recommend that if you ever get the chance, that you read it for all its worth - some have said it is hard to understand, because of the vocabulary or the ideas discussed, but I would say that it is totally worthwhile undertaking, even if you don't feel exactly in your comfort zone. You will benefit from its contents.
Source: Mere Christianity c. 1952 C.S. Lewis