This work explores the role of the Jewish Diaspora communities during the expansion of Christianity in the Roman Empire on the example of three cities: Alexandria, Antioch of Syria and Rome. Topics examined are characteristics of the Jewish communities in said cities, with special emphasis placed on their integration into the Greco-Roman world, communication with the majority population, and their relationship with the Roman authorities. This is followed by an analysis of the arrival of the new religion in said areas and the process of separation and independence of the Christian community from the Jewish community. The analysis is based on the information on the very beginnings of Christianity in the three cities, as well as on the information on the same Christian communities dating from a later period. By connecting said information with findings on the Jewish community up to that point of time, as well as through a mutual comparison of the centers, the attempt is to provide a more comprehensive image of the significance of the Jewish community for the first Christian communities. All three centers showed a strong relationship between the new Christian community and the Jewish community. It was visible in the substantial presence of Jewish Christians, and in the appropriation of Jewish community organization elements, as well as the literary and spiritual legacy of the Jewish diaspora. Additional conclusion is that the separation process between the Christian and Jewish communities at those centers proceeded without any clearly known great conflict.