i mean Paul called it "baptism" -- and i think he is referring to the Jews becoming 'disciples' of Moses & the law in this way.
so i don't think Paul's understanding of "baptism" is limited to literal ritual immersion in H[SUB]2[/SUB]O
now that you mention priest-washing --- i should dig around in the Midrash, but i'm pretty sure that the Levites regularly washed themselves ceremonially before eating, after touching anything ceremonially unclean, and fully immersed themselves before festivals and at other times. synagogues were built with mikvehs -- which are pools of water specifically for ceremonial washing, i.e. baptism. in the law, in addition to the ceremonial washing of Aaron and the other priests as they were ordained, there are many passages commanding ritual washing, including full-body immersion for anyone who had handled certain sacrifices.
i could go along with the washing of the priesthood as yet another baptism, into the temple ordinance.
washing with H[SUB]2[/SUB]O didn't "suddenly appear" with John -- it was an extension of the ceremonial practice in the Torah & Midrash, right?
so to 1st century Jews, "baptism" carried a great ceremonial significance - but it wasn't effective for washing sin, as Christ alluded to when He reprimanded the Pharisees for washing the outside of the pot & leaving the inside dirty. the inside is what must be cleansed, and only the Lord can do that.
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Well, I hate to speculate. But I have heard it said that gentiles had to be baptized to join Israel and that priests upon their consecration were baptized (I don't mean just washed, but immersed; I don't refer to the laver of the Tabernacle which I am confident was not for immersion). The gentile immersion is not in the Bible. And as to the priests, I have yet to check that one.
But surely John the Baptist had a novel ministry, calling Israelites to repent & be dunked.