What makes a novel "a classic?"

In my opinion what makes a novel a classic is one that has a significant meaning and literary value. In order for a novel to be a "classic novel" it would have to be unique in its own way. For example many of the plays written by Shakespeare are classic because Shakespeare wrote in a unique way. He wrote in a poetic tone and in prose when necessary. A classic novel would also have to have significant themes and motifs. Many people in today's society can relate to the situations that happen in many of Shakespeare's plays. So classic novels would have to have universal themes, something that everybody that reads can relate to.