This Salon article throws some light on many factors that make a book or a novel a classic. According to a Goodreads discussion thread on the same topic:
It has stood the test of time. It is filled with eternal verities. It captures the essence and flavor of its own age and had a significant effect on that age. It has something important to say. It achieves some form of aesthetic near-perfection. It is “challenging” or innovative in some respect. Scholars and other experts endorse it and study it. It has been included in prestigious series, like the Modern Library, Penguin Classics or the Library of America, and appears on lists of great books.
Not everything is classic, as you often see on television when they are showing a black-and-white movie (some of them are horrendous despite being black-and-white and despite featuring top-rated actors and actresses of the time) they say it is a classic. The same can be said about books. It is often said that there is no set formula for a book becoming a classic or a movie becoming a hit. Keeping some in the mental factors common, it is a random phenomena. For every War and Peace and Crime and Punishment there must be thousands of other books equally well-written and relevant but never saw the light of the day or never reached their intended readers. I often wonder why Shakespeare is such a reputed bard or playwright and when I read The Count of Monte Cristo I didn’t have a very high opinion of Alexandre Dumas. Similarly among Indian authors I really feel that writers like Premchand, R. K. Narain and Ruskin Bond are overrated. An author who sells a lot doesn’t necessarily become a good writer, but I think I’m entering another plane. Amritlal Nagar on the other hand, my recent discovery, is an exceptional writer and he can be easily compared to the best in the world and if I am in a position to compile my list of classics, I’m definitely going to add one of his books, especially Karvat.
There is a scholarly way of deeming a book classic and there is a personal way and I think whether a book becomes a classic all over the world is a mixture of scholarly and personal acceptance as well as a matter of chance.