Sunday's Gospel (Luke) is the first time baptism shows up in the Bible. So John's baptism of repentance is something new? Sort of, but not exactly.
God's people had been doing all kinds of ceremonial washing (baptisms) for centuries. Washing things to make them ritually clean. Washing Gentiles who converted.
But whatever/whoever was being washed, it was always about the outside. Dealing with something outside that was keeping whatever/whoever from being clean.
John's baptisms are different. They're about repentance, about cleaning people up where it really matters.
On the inside.
More on this tomorrow. Readings for Sunday