The 8th century was the start of the Gaonic Era in Jewish history. The great Jewish academies of scholarship and theological study in Babylon had continued to thrive since the completion of the Babylonian Talmud. These schools had grown in importance with the spread of the Jewish Diaspora over the world. Rabbis and exilarchs, Jewish community leaders in non Jewish lands, would send them questions on Jewish law and how to interpret ancient scriptures to apply in contemporary situations. The geonim, or geniuses, would debate these questions publicly, or put them to students as a test of their skills. The answers were recorded and sent back as Responsa.
Geonims were professors, supreme court justices, and religious arbitrators, though the position was achieved through longevity not merit. Most gaon over the four centuries of the era came from just seven Iraqi Jewish families. Of the many men, only a few dozen were truly exceptional and well remembered for their significant contributions to Judaic law and writing. Over time the success of the academies’ first goal of spreading the Talmud and Talmudic study made reliance on the gaon less necessary. Rivalry between the schools at Sura and Pumbedita and decreased financial support from the Diaspora weakened their standing as the centuries passed.
Life for Jews under the Umayyad caliphate was much like life under the Rashidun caliphate. They were second class citizens but probably not severely persecuted. Evidence suggests the Muslim minority in power depended on a large, higher taxed population of non Muslims to support the treasury, and this was one reason mass conversion was in fact discouraged. Dhimmi status was chosen by the Muslims as a tactic to prevent rebellion and uprising from conquered people, so they at least must have considered it preferable to other contemporary approaches.
To the east in the Christian Byzantine Empire, Leo III did not have the same feeling. After a narrow victory against Muslim invaders the devout emperor decided the problem with his kingdom was its religious plurality. In 722 Leo ordered that all Jews and Montanists be forcibly baptised. Montanists were a prophetic branch of Christianity, which held that women could be bishops, the Holy Spirit could speak through its prophets, and generally followed ascetic practices such as fasting. Three Montanist martyrs locked themselves inside their houses of worship and burned them to the ground rather than be baptised, but it did not inspire any uprising among their coreligionists.
Many Byzantine Jews fled to Bulgaria, but many others stayed. Most of these forcibly baptised Jews continued to still be Jewish in their hearts, and to practice Judaism secretly in their homes. Jewish mothers created “depatism” rituals, to wash away the dedication to Jesus Christ another had imposed upon their children. Men would stand guard outside the home while women made matzahs as they sang special prayers. Parents taught their children both their Jewish heritage, and how to hide their connection to it. These were “crypto Jews.”
Because of the endless campaigns of persecution against Jews for simply retaining their Jewishness, and because of their strong desire to continue doing so in the face of that, Jews have had to adapt. There is a Hebrew word anusim. In simple terms it means “forced ones”. In detail, this explanation from Jewish Virtual Library says it best. “Persons compelled by overwhelming pressure, whether by physical threats, physiological stress, or economic sanctions, to adjure Judaism and adopt a different faith.” Many anusim were were crypto Jews.