Judaism is a monotheistic religion based on the ancient teachings of the Torah and the Talmud. A follower of Judaism is a Jew.
Jews are as much an ethnic group as a religious one, and in the medieval period they were the perrenial outsiders. While many Jews lived in Christian Europe, they were reviled as an alien people, and slandered as the killers of Christ. Other falsehoods commonly spread about Jews included the murder of children and the poisoning of wells.
Jews suffered serious inequalities in law, taxation, and basic rights. Jews were forced to live in ghettoes (separate sections of cities) severely restricted in what jobs they could hold, what clothes they could wear, and whom they could associate with. Jews were often forced to wear distinctive badges or hats, and were not permitted to intermarry with Christians. They often suffered grievous punishments simply for being Jewish. Occasionally, dreadful slaughters of Jews called pogroms occurred, which were frequently no more than a combination mass riot and lynch mob bent on slaughtering every Jew they could find. Other, more organized forms of murder occured, such as the Spanish Inquisition, which, along with heretics, searched for "secret Jews".
While many Christians despised Jews, their presence was regarded as a "necessary evil", since Christians were forbidden the practice of ursury, the lending of money at interest. In period, Jews were often moneylenders, bankers and financiers. Many Christian nobles would borrow money from Jewish fianciers to pay for weapons, wars and even the Crusades. Some Kings even maintained the position of the Royal Jew, a sort of official court moneylender. However, such pragmatic and even-handed treatment of Jews in period was definitely the exception, not the rule.
Your SCA Persona is a Jew
The unofficial motto of the SCA, the Middle Ages as they should have been, is largely inspired by the treatment of Jews in period. While there are many SCAdians with Jewish personas, no one is interested in recreating the harsh and brutal treatment of the Jews. SCA conventions allow for Jewish personas without the unfortunate consequences that being a Jew in the Middle Ages would entail.