I grew up Roman Catholic, in a Roman Catholic household, under the influence of Roman Catholic parents who had both converted from different branches of Christianity. My father has two Ph.Ds, at least one of which is in Theology; I grew up going to Church, hearing my father discuss the Roman Catholic faith. I was homeschooled, and used a conservative Catholic program for high school, and I later went to a Catholic university for my undergraduate education: from age 14 onward I was introduced to thick theology textbooks, apologists like Augustine and Aquinas, and to varying religious/sociological/philosophical worldviews. That stuff is hard.
That's why I love C.S. Lewis. That's why I loved Mere Christianity. C.S. Lewis is, I believe, one of the great Christian apologists. (Wait: if I grew up specifically Roman Catholic, why am I interested in just a Christian apologist?!). What makes C.S. Lewis such a fantastic apologist--what makes Mere Christianity such an amazing book--is the man's ability to take the complex aspects of Christianity and boil them down to the simplest sentences.
I spent four years with Theology textbooks: I loved them, and I learned a lot about them. I spent around five years discerning a religious vocation with Poor Clares in rural Indiana: I loved it, and I learned a lot from those selfless, religious women. C.S. Lewis spent the last chapters of his book talking Christian Theology...and suddenly things that have been a part of my life, of my spirituality since I was thirteen years old and writing to a young Poor Clare, or since I was fourteen years old and working my way through my introductory Theology textbook, suddenly clicked and made sense in a way that they hadn't yet.
Why am I going on and on about a Christian apologist and a book called Mere Christianity when I grew up with such a thorough religious background?
Because I'm probably not the average Christian. (No offense to anybody!). Not every Christian is Roman Catholic, and not every Christian has some of the advantages of learning their religion from converts. Most definitely not every Christian has a Theology professor as a parent, and many Christians have--for some reason or other--not had an education offered through a Christian standpoint. I did have all those things, and C.S. Lewis has helped me to understand my faith.
Imagine how much he can help people who are new to this! Imagine how much more sense he can make to people who aren't carrying four years of Theology textbooks, eight years of Christian education, and a lifetime of religious influences with them!
And I think the fact that C.S. Lewis is NOT Catholic, but merely Christian, is an important fact. He was of the Church of England...but he didn't call his book Merely Anglican or Merely Protestant...he called it Mere Christianity. C.S. Lewis, while having his own affiliation, has an amazing ability to step outside of his own religious denomination and affiliation in a way that a lot of people can't; the fact that C.S Lewis is not Catholic is a bonus point for him in a world that is still full of denomination and affiliations.
NOTE: Don't get me wrong--I am a staunch Catholic, and I firmly believe in my faith as it has been handed down to me; I'm not Catholic because I'm just too lazy to explore any other denomination, but because I truly believe I'm in the right place.
But the above note doesn't change the fact that sometimes we Catholics--or Protestants, or Anglicans, or Lutherans, or non-denominationals--oftentimes get so wrapped up in talking about our specific dogmas, etc., that we forget to delve down and show the world the basics. We're so focused on public declarations about Mere Catholicism or Mere Lutheranism or any other form of Mere [insert denomination/affiliation] that we forget to tell the world about Mere Christianity.
Luckily for us, C.S. Lewis has done it for us. So when someone says 'I don't want to hear about your specific stuff, I just want to know the down and dirty, the basics, the 101 of Christianity'...feel free to explain it, to share your experiences and your viewpoints. But offer a disclaimer before you do so, and make sure to point them to C.S. Lewis's Mere Christianity at the end of it.