If you're already a fan, you might get what I'm going for, but if not, indulge me this passage.
'"But why couldn't Quirrell touch me?"
"Your mother died to save you. If there is one thing Voldemort cannot understand, it is love. He didn't realize that love as powerful as your mother's for you leaves its own mark. Not a scar, no visible sign... to have been loved so deeply, even though the person who loved us is gone, will give us some protection forever. It is in your very skin. Quirrell, full of hatred, greed, and ambition, sharing his soul with Voldemort, could not touch you for this reason. It was agony to touch a person marked by something so good."'
I've been in churches that didn't much care about the non-Christian world, though to greater or lesser degree they might have paid lip service. But for the most part, how Christians are to interact with the non-believers around them is a pretty important topic for most churches. Of course there are things like love, kindness, etc. but I'm thinking particularly of how we interact in regard to culture. Ours [American, western] is a "culture of death," a culture that serves Mammon, whose god is its stomach, etc. There's a lot of bad to be said about our culture. I know a lot of Christians who think the best response to this is to become more and more insular - homeschooling, watching only Veggietales even into their teens (because Disney is too racy) and Fireproof after getting married, listening only to "Christian" music, etc. But there are a lot of problems with this (not least of which is that the observant Christian will find the occasional treasure within our culture).
On the other hand, I know quite a few people who insist that "relevance" is important. Christians should be familiar with pop culture, perhaps even hyper-familiar. Personally I tend to lean a bit more to this side, although not as far as most. There's a real danger here because any culture in which we participate shapes us, and I've known far too many Christians who, little by little, become completely unremarkable people. Perhaps they retain a few odd customs like going to church, but that's it. Or even that may get left behind.
Which brings me back to the quote. I believe there is no more powerful message to the culture around us than to be ourselves. The more truly and completely we live out the goodness of the God who created this world, who gave us a place in it, and who redeems us, the more we are different from what the world around us is but more like what it knows (deep down) it needs. Christians marked by the love and goodness of God are painful to be around when you don't feel good and you can't remember (or never knew) what love was like. But it's a good kind of pain, a kind of pain you can't stay away from. It's a beautiful agony.
Of course I know this is oversimplified. Too often we cause our neighbours agony of an entirely different kind. Or even just the annoyance that normal human beings cause each other. Rather than being good we are goody two-shoes. Or just pretend to be good but hold on to ugly and selfish attitudes, thoughts and behaviors in secret.
In real life, being so good it hurts to be around gets complicated, and it's not easy. At the very least, it's kind of exhausting! I mean, to approach every encounter - whether riding the bus or talking to my neighbour or going to work every day - without protecting myself or my ego, without seeking something from others but seeking to give to them, without getting annoyed when my own plans are disrupted...In fact, I don't think we can do it! I think that's why we need and why we have been blessed with the spirit of God. So I think the work of Christians in the world around them is to live following God's spirit in every moment.
We do that individually and together, and even when we do it individually we need each other for strength, but that's another topic.
Recommended Reading: Sticking with C.S. Lewis, I heartily recommend Out of the Silent Planet and Perelandra. (Actually, I recommend the whole trilogy, but not for now, and I'm really only recommending the first so the second makes a bit more sense.) In Out of the Silent Planet Lewis ponders our planet's condition of (what theologians call) "fallenness" and imagines pretty much the rest of the universe continuing on as God had intended. This leads a space-traveling character, as well as the reader, to ponder fallen humanity, modernity, the post-war West. Perelandra resumes these themes and paints a great picture of Edenic life (and resurrected life). It's a great work of Christian imagination and one of my favorite books. If you've ever thought that sitting on clouds strumming harps sounds boring then (#1 congratulations for not stupidly buying into churchy nonsense, and #2) this book is for you.
Although your bookstore probably has these under "science fiction" they're really theological novels...only bookstores don't recognize that as a category.