When we talk about "messages" of an author in any text, the word we typically use is theme. So what are some of the themes, or overall messages in this short novel? Despite its length, there are many.
One message Charles Dickens conveys throughout the novel is the idea of not taking anything for granted. As Scrooge is able to travel through his life and look in on the past, present, and future, as an outsider, he sees how stingy he has acted toward others. He also sees, in the Cratchit family, how to be thankful for the small things. As a result, his mind and behavior changes. One common reaction to this from readers is to consider a similar circumstance in our personal lives. What if we were given the same opportunity as Scrooge was given? What would we see? What would we regret, and then wish to change for the future? I think many readers react with a similar sense that humans often take many blessings for granted and forget to be gracious.
The above theme lends itself to another, and that is the message of self reflection. The entire story is one of self-reflection, and the audience travels with Scrooge through his physical and emotional journey of change. Again, it is clear that Dickens means to speak to a larger crowd than just Scrooge. It is subtly suggested that perhaps there is a little bit of Scrooge in everyone, but if we would simply take some time (one night, even) to self-reflect, perhaps we could ultimately make the world a better place.
For more ideas on theme in this story, see the reference link below.