baptism of fire vs christening

baptism of fire

noun
  • The gift of the Holy Spirit. 

  • Martyrdom by immolation. 

  • Tribulation endured as spiritual discipline. 

  • The first experience of a severe ordeal, especially a first experience of military combat 

christening

noun
  • Any instance of someone's or something's being christened. 

  • The Christian sacrament at which someone, usually a child, is baptized and given a Christian name. 

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