John’s Baptism
When John the Baptizer begins his work, Jerusalem sends a fact-finding committee to to inquire about his identity and about his baptism. In particular, the Pharisees ask why he’s baptizing since he’s not the Messiah or Elijah or the Prophet.
But you’ll notice that they don’t say, “What is this curious ritual you’re performing down here at the Jordan? Why are you scooping up water and pouring on all these people’s heads?”
They know about baptism already. They just wonder why he is doing it. What does it mean when John appears in the wilderness, by the Jordan, and starts calling Israel out to be baptized there — at that spot, in that river — by him.
What does baptism mean in the Gospels? Joel Garver, in this old essay, “Baptism in Matthew and Mark,” gives the most helpful answer I’ve seen.