pessimism vs unbelief

pessimism

noun
  • The condition of being pessimal. 

  • A general belief that bad things will happen. 

  • The doctrine that this world is the worst of all possible worlds. 

unbelief

noun
  • An absence (or rejection) of belief, especially religious belief 

How often have the words pessimism and unbelief occurred in a corpus of books? (source: Google Ngram Viewer )