Judaism is both a religion and an ethnic identity. Its religious component is predicated on the Hebrew Bible and other literatures, and is a monotheistic faith. As an ethnicity, Judaism embodies the culinary and literary cultures of Jews throughout history. Many Jews today are not religious, but see themselves as part of the Jewish people.
Questions tagged with judaism should refer to the ethnicity/religion of people who are identified as being Jewish. Questions about those people themselves should be tagged as jews.
To refer to the ancient Roman Empire provinces of Iudaea or Palestina, consider using palestine along with roman-empire.
For the genocide of Jewish people during the ww2 use the holocaust tag.
For the modern-day state of Israel, it is more appropriate to use israel or palestine, depending on the question's focus.