baptism by fire

baptism by fire AND baptism by fire (an ordeal or martyrdom. More recently, a soldier's first experience of battle.) — крещение огнем; || боевое крещение

Origin

The term refers back to actual martyrdoms by fire. The meaning most often used now is of a soldier's first experience of battle. Baptism because battle is new to him and 'fire' from the firing of guns, i.e. he is 'under fire'.