What is the theme in Hogfather by Terry Pratchett?

The main theme in Hogfather is the importance of believing. The auditors hate humanity and want to destroy it. They know the best way to destroy humanity is to rid the world of the Hogfather, who is a significant belief though a small one. If the auditors can destroy the belief in Hogfather, then it becomes harder for people to believe in other things that aren't tangible such as justice, mercy, and love.