Baptism of Fire

Someone is said to go through "a baptism of fire" if they're launched into a new job or a new situation in a very abrupt way. To change the metaphor, they're thrown in at the deep end. So where does the original baptism of fire come from?

John the Baptist was a fiery preacher who was active right at the start of Jesus' public ministry. He baptized people in the River Jordan, and famously predicted that someone even more fiery than him was about to arrive on the scene...

I am just baptizing with water. But someone more powerful is going to come, and I am not good enough even to untie his sandals. He will baptize you with the Holy Spirit and with fire."

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