We have seen how the state attempted to control the infiltration and adoption of foreign cults. A monotheistic religion, however, could not be reconciled with traditional Roman religious practice, as the above excerpt from Tacitus implies. The first extant reference to the presence of Jews in Rome is to their expulsion from the city in 139 BCE because they were trying to pass on their sacred rites to the Romans (Valerius Maximus, Memorable Deeds and Words 1.3.3). The numbers involved, however, were probably quite small, whether permanent residents or transients.

The sources for the next several decades are scanty, but by the late republic there was a substantial Jewish community permanently resident in Rome. Most of these people had been brought to Italy as captives, probably after Pompey's conquest of Syria in the 60s BCE. Many had been freed and later became Roman citizens (Philo, Embassy to Gaius 155). The Roman authorities were consequently faced with the permanent presence of people whose ancestral customs and conventions were very different from their own.