religion vs unbelief

religion

noun
  • Rituals and actions associated with religious beliefs, but considered apart from them. 

  • Any practice to which someone or some group is seriously devoted. 

  • Belief in a spiritual or metaphysical reality (often including at least one deity), accompanied by practices or rituals pertaining to the belief. 

  • A particular system of such belief, and the rituals and practices proper to it. 

  • The way of life committed to by monks and nuns. 

verb
  • Engage in religious practice. 

  • To make sacred or symbolic; sanctify. 

  • Indoctrinate into a specific religion. 

unbelief

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

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