Complex sentence: Long ago, Jews went underground from the repeated pogroms against them, continuing to practice their religion in secret, under cover as "religious revivalists," to conceal their unbroken connection to ancient history.
Split into simpler sentences: Long ago, Jews went underground to escape the repeated pogroms against them. They continued to practice their religion in secret, under cover as "religious revivalists," to conceal their unbroken connection to ancient history.