Other historians believe that conversion during the Roman era was limited in number and did not account for much of the Jewish population growth, due to various factors such as the illegality of male conversion to Judaism in the Roman world from the mid-2nd century. Another factor that would have made conversion difficult in the Roman world was the halakhic requirement of circumcision, a requirement that proselytizing Christianity quickly dropped. The Fiscus Judaicus, a tax imposed on Jews in 70 CE and relaxed to exclude Christians in 96 CE, also limited Judaism's appeal. In addition, historians argue the very figure (4 million) that had been guessed to account for the population of Jews in the ancient Roman Empire is an error that has long been disproven and thus the assumption that conversion impacted Jewish population growth in ancient Rome on a large scale is false. The 8 million figure is also in doubt as it may refer to a census of total Roman citizens.