The origins of the Ashkenazim are obscure, and many theories have arisen speculating about their ultimate provenance. The most well supported theory is the one that details a Jewish migration through what is now Italy and other parts of southern Europe. The historical record attests to Jewish communities in southern Europe since pre-Christian times. Many Jews were denied full Roman citizenship until 212 CE, when Emperor Caracalla granted all free peoples this privilege. Jews were required to pay a poll tax until the reign of Emperor Julian in 363. In the late Roman Empire, Jews were free to form networks of cultural and religious ties and enter into various local occupations. But, after Christianity became the official religion of Rome and Constantinople in 380, Jews were increasingly marginalized.