NO ONE really knows what qualities turn a book into a classic. Cynically, you could say that a classic is an old book that is still in print. The longer it is since the book was published, the more reliable the judgement that has given it classic status.
It is a process not very different from the ordinary processes of publishing: it is still a judgement made by the market. But as time goes on, the mistakes made by a book's first audience drop away, both the enthusiasm for something banal and the bafflement at something interesting and new.
What is left is what we have agreed is a classic; how long that process takes nobody knows. Certainly hundreds of years, perhaps thousands. There are fashions in ancient literature as in anything else, and some authors rise in status as others fall from sight. Donne is a part of the living culture, a classic now as he was not a century ago; and, as if to make room for him, other, once highly regarded names are shuffled off to the stacks.
A century ago, the idea that an author such as Statius would soon be forgotten would have been regarded by any educated person as utterly incredible. Academics have started to comment that that means there are no classics, but inasmuch as, notoriously, most of the newer sorts of academics have never read anything at all, I think we can disregard their opinion.
It is a long process, and it is always tempting to start sorting the sheep from the goats early. Penguin is relaunching its splendid Modern Classics series with a few books from the very recent past. Jane Bowles and Paul Bowles; Nancy Mitford; Vladimir Nabokov; and, most surprising, Martin Amis's Money. I bought Money when it came out, and it does not seem all that long ago; it is startling to see that it has now, without one quite noticing it, become a classic of the literature.
All the same, Amis's book certainly has a strong claim to classic status; the music of its sentences rings in the ears of half of the novelists writing today in Britain. Whether it will last is another question and not one that we can reasonably provide an answer to. You may choose to think that the label of "classic", when applied to books not 20 years old, is just another marketing tool. No book became a classic because a publisher called it one; a book becomes one when thousands of readers, over the course of decades and centuries, continue to enjoy it and take an interest in it.
However, I think the label retains some of its force. If hypocrisy is the homage that vice pays to virtue, then …