For at least a century before the establishment of the Augustan principate, Jews and Judaism were tolerated in Rome by diplomatic treaty with Judaea's Hellenised elite. Diaspora Jews had much in common with the overwhelmingly Hellenic or Hellenised communities that surrounded them. Early Italian synagogues have left few traces; but one was dedicated in Ostia around the mid-1st century BC and several more are attested during the Imperial period. Judaea's enrollment as a client kingdom in 63 BC increased the Jewish diaspora; in Rome, this led to closer official scrutiny of their religion. Their synagogues were recognised as legitimate collegia by Julius Caesar. By the Augustan era, the city of Rome was home to several thousand Jews. In some periods under Roman rule, Jews were legally exempt from official sacrifice, under certain conditions. Judaism was a superstitio to Cicero, but the Church Father Tertullian described it as religio licita (an officially permitted religion) in contrast to Christianity.

What means guaranteed the Jews and Judaism in Rome?