Mere Christianity is widely recognized as a classic of the Christian faith. Lewis appeals to a broad range of Christian traditions, from theological conservative to liberal. Originally given as radio addresses during WWII and a few years following, C.S. Lewis tried to explain and defend the basics of the Christian faith. I have read this book more times than any other book outside of the Bible (perhaps five or six times now). On this reading I realized two things; First, I can see where much of my own thinking and preaching has been deeply influenced by Lewis in general, and Mere Christianity in particular. Second, as I grow in my own understanding of theology, I realize more and more where I disagree sharply with some of Lewis' thoughts on Christianity. For example, Lewis' philosophical and apologetic reliance on his view of 'free will' is much more arminian in thinking than mine. In addition, Lewis seems to presuppose the validity of human evolution, as well as a kind of universalism through salvation of adherents of other religions if they are sincere (something he also implies in The Last Battle in The Chronicles of Narnia Series).
Some have even argued that Lewis is very much like Rob Bell in these areas. However, these are not focal points or even essential teachings of Lewis (whereas it seems that Bell is doing just this, or at least capitalizing financially on these issues). Furthermore, there is much to be gained from Lewis' thinking and apologetics. Therefore I would continue to encourage believers and non-believers alike to read carefully through this classic book.