being forced into a difficult situation and having to learn by doing
—Because she missed the training session, Donna felt as if her first day working at the factory was a baptism of fire.

The John who announced the coming of Jesus was called John the Baptist because he baptized those who wanted to turn away from their sins to get ready for Jesus’ arrival. But he said that the baptism that Jesus would bring was greater than his:

I baptize you with water, for repentance, but the one coming after me is more powerful than I am—I am not worthy to carry his sandals. He will baptize you with the Holy Spirit and fire [baptize you with the Holy Ghost, and with fire]. (Matthew 3:11)

Baptism comes from a Greek word meaning “to dip, immerse.” There are several views as to what the “fire” in this verse represents, including judgment, suffering, purification, and the power of God’s Holy Spirit.

In the past, baptism of fire was used to refer to the death of a Christian martyr and later to the first time a soldier takes part in a battle. Now it used for any experience in which soemeone learns a hard lesson by experiencing difficulty.