expiation vs propitiation

expiation

noun
  • An act of atonement for a sin or wrongdoing. 

propitiation

noun
  • The act of propitiating; placation, atonement, similar to expiation but with the added concept of appeasement of anger. 

  • The death of Christ as a basis for the forgiveness of sin. 

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