Change your sinful, pagan ways, and do it in public. People so desperate to have an intellectual behind them will buy your book in bulk.
I don't get C.S. Lewis. I never read the Narnia books; I tried to watch one of the movies. I failed. I haven't read The Screwtape Letters. I've browsed Mere Christianity. It's got some funny parts, but Lewis's approach to satire is too brutal even for me. If he's posing as a convert to Christianity, the book is insulting to Christians in ways I would never attempt because for all my discomfort with and occasional disdain for religion I do respect genuine faith and people who are inspired to do good in the world because of it; if his arguments are meant to be taken seriously, then Mere Christianity has got to be one of the weakest books ever written by someone we're supposed to regard as intelligent.
I normally believe you should read a book before you try to make grand claims about it, but if growing up Catholic taught me anything it's that there's always a loophole. And if people can call themselves Christians without reading the Bible or even attempting to follow Jesus's teaching, I can say, without having completed it, that Mere Christianity is a stale load of shit covered in moldy vomit sauce with a side of overdone donkey balls.