"A great classic means a man whom one can praise without having read." - G.K. Chesterton
It's virtually a cliche to point out that Chesterton is among the most quotable of authors. But it's easy to misunderstand the Chesterton quote taken out of context. For instance, take the quote above, from his essay "Tom Jones and Morality" in All Things Considered. Our first reaction to it may be to think that GKC is being ironic and taking a swipe at people who talk up a classic without having read it. But in context it is clear that GKC means no such thing and intends just what he says.
Chesterton's point is ultimately conservative in the best sense of the word. A great classic becomes so based on the developed opinion of mankind over many decades or centuries. We can praise a classic without having read it based on trust in that common, longstanding opinion. I can, Chesterton says, talk of "great poets" like Pindar without ever having read Pindar because "a man has got as much right to employ in his speech the established and traditional facts of human history as he has to employ any other piece of common human information." And the status of great classics is one of those "established and traditional facts.
While GKC defends the right of men to praise a classic without having read it, he disputes a right to condemn a classic without having read it. The reason should be obvious. Praising a classic is submitting to the historically developed consensus concerning a work; condemning one is contradicting that tradition and, so, going it on your own. If you are going to contradict the received opinion, you've got to have some reasons for doing so, and it is hard to see how you could have good ones without having read the work in question.
GKC never wrote pithy quotes for the sake of being quoted. His wit is always a spur to more considered reflection - a reason for us to be careful of a GKC quote absent context.