 A baptism of fire means an intense introduction to something. This began in the 16th century with Protestant Christian martyrs. They were burned at the stake by Catholics who believed that the practice gave the martyrs a form of baptism before they were judged by God. Napoleon was the first to use the expression, and he used it in French, which I won't attempt, but it meant a baptism of iron. But the phrase is now used to apply to military situations and a soldier's first experience of war.