baptism of fire

WordReference Random House Unabridged Dictionary of American English © 2025

bap′tism of fire′, 

1. spiritual sanctification as a gift of the Holy Ghost.
2. the first time a soldier faces battle.
3. any severe ordeal that tests one's endurance.
• 1815–25

Collins Concise English Dictionary © HarperCollins Publishers::

baptism of fire noun

1. a soldier's first experience of battle
2. any initiating ordeal or experience