Although Adam followed God in the Old Testament, and therefore could be counted as Jewish in a sense, when did Judaism as we know it from the Old Testament (with a priesthood, temple and sacrifice) come about? Was it Abraham?
Abraham and the People of the Covenant, who came to be called Hebrews and much later Jews, had a Prophet-centered belief system. That was maybe 400 ye rs or so? Then there were Judges. During this time, the Levites were designated to be priests.
No Temple.
As for Sacrifice, EVERYONE had sacrifice, all religions practice some form of sacrifice. The hallmarks, or unique characteristics, of the Hebrews were the Covenant, One God and the Law. The thing that has always been part of the People of the Covenant under any name is
the Law.
Anyway, about 700 yrs in, after they get to Canaan, they ask for a King. God doesn’t think this is a good idea, but they insist and so they start having Kings and almost immediately break into the Northern and Southern Kingdoms. So, the Kingdom of the Judeans gives us the word “Jew” or “Judaism.” The Northern Kingdom is decimated, they are all run off and start their own scattering of communities, but they still have the Law and the priests and the Covenant. Then, later on, the rest of them get scattered, with the largest number being in Babylon, where they did quite well.
See, their conquerors never insisted they stop being Jews, so they just regroup. It was during the last Babylonian exile that, scholars believe, the writings of the two Kingdoms were combined into the Scriptures we have today.
When the Jews were allowed to return to Jerusalem, a lot of them didn’t go, they had a good thing in Babylonia. Some zealous sorts went and started rebuilding the Temple. There was also a Temple in Egypt, BTW, which had about a 40% Jewish population.
So, most of the Jews were actually NOT in Jerusalem/Palestine and most of them never saw the Temple, like most of us will never see the Lateran Palace.
After the final razing of the Temple in 70AD, there would have been probably 3 million Jews in various places all over what we call the known world, all connected by the Law, by the culture and practice of their faith and still, 1500 years after Abraham, still the People of the Covenant. They then moved to a 4th sytem, called Rabbinical Judaism, which is pretty much, I think, what we still have.