The supernatural is one of the strongest and most prevalent themes in A Christmas Carol. In fact, in his preface to the book, Dickens calls the story "this Ghostly little book" to empathize the theme's importance.
Beginning with some examples, we find evidence of the supernatural from the very first stave. Scrooge's door knocker, for example, comes hideously to life in the form of his dead associate: "...with ghostly spectacles turned up upon its ghostly forehead. The hair was curiously stirred, as if by breath or hot-air; and, though the eyes were wide open, they were perfectly motionless."
Scrooge's ghostly visitors possess many supernatural qualities, too. Take the Ghost of Christmas Past in stave two, for example: "He turned upon the Ghost, and seeing that it looked upon him with a face, in which in some strange way there were fragments of all the faces it had shown him, wrestled with it."
These supernatural elements have many important purposes in the book. They help Dickens to create strong, often grotesque, characters, and an even stronger sense of atmosphere. They also serve to drive the plot: we know, for example, that every time the clock chimes, Scrooge will receive another ghostly visitor. These elements also engineer Scrooge's transformation. Without seeing the visions of the past, present and future, Scrooge would never have the impetus to change and reform his character.
The supernatural elements also illustrate Dickens' interests, outside of writing. He was intrigued by ghosts and this "other" world. He practiced spiritualism, performed magic tricks for his family and friends and wrote many articles on the supernatural for his journal, Household Worlds. A Christmas Carol, thus, enabled him to explore these interests in another way and its success would inspire him to continue this trend in many of his later works.