Let’s forget about C.S. Lewis’ religious philosophy for a moment.
Let’s say you’re a completely secular idealist trying to win support for a cause you consider obviously virtuous. It doesn’t matter whether you’re calling for legalized gay marriage or the abolition of abortion, whether you want to reduce global warming or cut taxes. What matters is, you have a cause that you believe is righteous, you’ve spent a long time thinking about it, and have what you think are important things to say on the subject.
You stand up before a crowd to speak your mind. You go in knowing that, if you say something stupid, people are going to laugh at you, but you figure you’re ready for that. You do a lot of scrupulous research, you go over your notes repeatedly, and you’re sure there’s absolutely NOTHING in your presentation that could possibly inspire a smirk, let alone ridicule.
But… as you start to speak, you notice something: much of the crowd is ALREADY smirking, before you’ve said a thing! Your every statement draws snorts and snickers, even when you KNOW you haven’t said anything foolish or even incorrect. You try to keep your composure, and say, “I seem to have said something hilariously wrong. Would someone be kind enough to set me straight and tell me what it is?” But nobody tries to engage you- they just continue to snort and snicker for the duration of your talk. And when you leave, angry, you hear them saying that you were a pompous stiff with no sense of humor.
“Flippancy,” as Lewis uses the term, describes the attitude of the crowd. A normal crowd of people with ordinary senses of humor is always ready and willing to laugh, as soon as they see/hear something funny. An ordinary crowd is prepared to laugh hysterically when Jerry Ford slips on a banana peel, when Joe Biden makes a silly gaffe, when Dan Quayle trips over his words, when Ted Kennedy babbles on in a drunken stupor.
A flippant crowd, on the other hand, doesn’t wait for you to say anything stupid. It comes in ASSUMING you’re stupid, and interprets everything you say to confirm that assumption.