What turns a novel into a classic? There are at least three attributes that are consistent with every classic: Significance, relevance, and time.
When determining whether or not a novel is in fact a classic, significance is well, highly important (do you see what I did there?). A classic novel per se should be significant to its time period. It should stand out as a testament to its time period's history and epitomize the modus operandi. Dickens' Oliver Twist is a social novel depicting the horrors of child labor and poverty during the industrial revolution, Upton Sinclair's The Jungle depicts the same horrors needing much reform in America. Both novels are still studied and read today because of their historical significance. The lessons their authors taught changed the way the public viewed poverty and proved to be catalyst to social reform.
Do the lessons the novel teaches us relevant today? By far, no author or playwright is quoted more than Shakespeare. Disney's 1994 classic, The Lion King is loosely based on Shakespeare's arguably best play, Hamlet. If the subject of love could be put into words, those words would undoubtedly come from Romeo and Juliet or perhaps one of his sonnets. To name drop another classic work is Sun Tzu's The Art of War which was mentioned in Oliver Stone's Wall Street. These lessons are still relevant. Why?
Because they've withstood the test of time. The last and final attribute is Time. The worst thing that can happen to a book is for that book to become Out of Print. To a book, Out of Print is blasphemy. Should a book fall to this fate, significance and relevance would be its reasons. The book failed to achieve the two. I'm sorry to say that there are far more books in human history that are out of print in contrast to books that are in print. I don't know the exact number but I can guarantee you that it's not close.
Only the most special books can become classics. Only the very best, most reviewed and most sold books last. Period.