Meaning: A novice has his first experience in combat, or in some other stressful situation.
Psalm 66:12
Thou hast caused men to ride over our heads; we went through fire and through water: but thou broughtest us out into a wealthy place.
Isaiah 43:2
When thou passest through the waters, I will be with thee; and through the rivers, they shall not overflow thee: when thou walkest through the fire, thou shalt not be burned; neither shall the flame kindle upon thee.
Matthew 3:11
I indeed baptize you with water unto repentance: but he that cometh after me is mightier than I, whose shoes I am not worthy to bear: he shall baptize you with the Holy Ghost, and with fire:
Luke 3:16
John answered, saying unto them all, I indeed baptize you with water; but one mightier than I cometh, the latchet of whose shoes I am not worthy to unloose: he shall baptize you with the Holy Ghost and with fire: