In the days when John the Baptist baptized Jesus in the Jordan River, baptism was not completely uncommon. John was not doing a brand new thing, but at the time, Jews used baptism in only two ways:
First, baptism was part of the conversion process for Gentiles to become Jews. This process typically involved three things: you were circumcised (which I think had to reduce the potential male convert pool pretty dramatically), you memorized some key passages out of the law and you got baptized, showing that you were washing away your previous, sinful, pagan life.
The second use of baptism was a ritual cleansing you gave yourself, as a Jew, in purification ceremonies, before you offered sacrifices and the sort.
But John’s baptism was different than either of those. It wasn’t aimed at Gentiles; it was directed at Jews. So this wasn’t about becoming a Jew or just a ritual cleansing.
It was a baptism of repentance. He was talking as if Jews—religious Jews—needed to be converted, too. That was a revolutionary concept.
John noticed many Pharisees and Sadducees coming to his baptism, because that’s what religious people do: They find out where religious things are happening in the religious world, and they go hang around and do those religious things so they can add them to their religious resumes.
John said to them, “You brood of vipers! Who warned you to flee from the wrath to come? Bear fruit in keeping with repentance. And do not presume to say to yourselves, ‘We have Abraham as our father,’ for I tell you, God is able from these stones to raise up children for Abraham. Even now the axe is laid to the root of the trees” (Matthew 3:7 ESV).
John is telling the religious leaders to stop hiding behind their religion. They’ve been telling themselves, “I’m a Jew, and I’ve been through this or that ritual, so I’m OK.” What John is talking about—true repentance—is something that needs to happen at your very root.
One of the most revolutionary stories Jesus ever told is the story of the prodigal son.
The first son took his inheritance, left his father’s home and gave in to riotous living—today’s equivalent of frat parties, smoking weed and blowing all your money in Vegas. He was separated from his father because of his rebellion.