Judaism has never been a monolithic community. Moreover, the continuity of Judaism and the Jewish people is not the consequence of adherence to one way of being Jewish.
From the early days of the biblical Israelites straight through to modern times, Jews and their ancestors have preserved a group identity because of their willingness to adapt to new circumstances. Though this way of viewing Jewish history is often denied in more traditional quarters, it is the view that is endorsed by historians.
It is true that Jews have preserved many customs, some of them for thousands of years. It is equally true that even a very traditional Jew of the 21st century would barely recognize the Judaism of 2,000 or more years ago. And a Jew from ages past would be baffled by the varieties of Jewish possibilities in our modern age. This ability to evolve and adapt is what has preserved Jews and their various forms of Judaism for so many years.