In the first century Roman empire, "gods bumped up against each other with some frequency even as humans did" observed Paula Fredriksen in her essay "What Parting of the Ways?" As Rome expanded across the Mediterranean and Middle Eastern worlds, so did its plurality of gods and religious expressions. In imperial Rome, subjugated peoples were not expected to abandon their ethnic gods. Quite to the contrary, the very nature of empire (the subjection of diverse peoples over large areas) meant the inclusion of many peoples and their gods (or perhaps better, many gods and their peoples) under a single Imperial government.
The Roman empire was a world of religious pluralism with many divergent religious views and practices existing side-by-side and expressing the cultures of subjugated nations. This was not a system of religious tolerance - for tolerance assumes a single established, state-sponsored religion that nevertheless extends a degree of religious autonomy to dissenting religious groups. Rather than suppressing or tolerating the native religions of conquered peoples, the Roman imperial government positively accommodated these varieties of faith - as long as these religions met two fundamental criteria: antiquity and ethnicity.
Fredriksen, in a delightful turn of phrase, points out that ancient religion "ran in the blood" - that is, it was tied to particular peoples who lived in particular places. Native religious commitment was the "natural" and expected commitment of every member in a given society and culture.
The Roman government recognized the legitimacy of "foreign" religions by their antiquity and ethnicity. Ancient religions - those that had endured the test of time - were treated with respect and accommodation; whereas new religious expressions were deemed innovative, suspect, and potentially dangerous. Legitimate religions also expressed the "ethnicity", the culture, of native peoples. Religion bound people to a common culture, legitimating institutions and social structures and providing means of social control within the group. Such religions provided the backbone of social order. A religion that demonstrated antiquity and ethnicity - like the faith of second temple Judaism - enjoyed recognition and accommodation by the Roman government.
The connection of religion to national and social identity was particularly threatened by the destabilizing effects of religious conversion. Embracing a new religious idea or practice into one's a larger religious worldview and commitment was not a problem. But making an exclusive commitment to a new and different worldview meant abandoning one's native ethnic religion - a traitorous act against nature, an act that could only be disruptive to the social order.
These observations lead us to describe two types of conversion found in the New Testament writings: one which was largely ignored by Rome and the other which threaten the Roman order.
The first kind of conversion occurred within the Jewish faith when an individual abandon a specific interpretation of the Torah to embrace a rival Torah interpretation. The earliest Jewish Christian conversions were of this type. Christians abandoned the Torah interpretation of Hillel, Shammai, or Gamaliel in favor of the new Torah interpretation of Jesus. The conversion of Saul of Tarsus/Paul seems to be exactly this type of conversion from one Jewish school of interpretation to another.
As long as it was understood as just another variety of Judaism, Christianity enjoyed the Roman accommodation of this ancient and ethnic faith. Even the "God fearers" - those Gentiles drawn to the Jewish synagogue with its ethical monotheism - were not seen as a direct threat to the Romans. For the God fearers appeared only to "add" Jewish thought and practice to their own native beliefs, not submitting to the ultimate identity marker of circumcision which signified a complete conversion to Judaism - a radical change that abandoned native religious practices in favor of an exclusive commitment to the foreign faith.
But the Roman accommodation of the early Christians began to break down with the success of the Gentile mission. While the earliest Christian inroads in the major Roman cities of Syria, Asia Minor, and Greece were all tied to Diaspora Jewish synagogues, the eventual "parting of the ways" of the Gentile Christian converts from non-Christian Jewish synagogues and the growing number of thriving Christian "house" churches alerted the Roman officials to a different kind of conversion among the Gentile Christians. These converts were abandoning their native pagan religions - including refusing to participate in the civic devotion to the patron gods of the cities which was understood to guarantee the welfare of these municipalities. The Gentile Christian converts would not offer public sacrifices to the gods, nor participate in the temple ceremonies and public spectacles/parades for the gods, nor even eat meat from the marketplace that had been sacrificed to pagan deities.
The rejection of these identity markers of the pagan religions demonstrated that these Gentiles were "true converts," making an exclusive commitment to a foreign religion other than their native faiths. Over time, as Christianity and Judaism parted ways, it became clear that Gentile Christian conversion was not the embrace of legitimate ancient Judaism with its particular identity markers (circumcision, Sabbath observation, and dietary restriction), but rather - to Roman thinking - the introduction of a new religion - an innovation that was not to be confused with or accommodated to like ancient Judaism.
More and more, at the end of the first century and throughout the second and third centuries CE, the "parting of the ways" of Jews and Gentile Christians resulted in rising hostility by the Romans toward the Gentile Christian converts who were seen as traitors to their legitimate native religions and ultimately subversives to the Roman political and social order.