1
general::
A severe ordeal or test, especially an initial one, as in This audition would be Robert's baptism of fire. This term transfers the original religious rite of baptism, whereby holiness is imparted, to various kinds of ordeal. At first it signified the death of martyrs at the stake, and in 19th-century France it was used for a soldier's first experience of combat. Currently it is used more loosely for any difficult first encounter.
American Heritage Idioms
2
general::
Phrase(s): baptism of fire
Fig. a first experience of something, usually something difficult or unpleasant. • My son’s just had his first visit to the dentist. He stood up to this baptism of fire very well. • Mary’s had her baptism of fire as a teacher. She was assigned to the worst class in the school.
McGrawhill's American Idioms And Phrasal Verbs