Daniel, Jerry L. “Anti-Semitism in the Hellenistic-Roman Period“, Journal of Biblical Literature, Vol. 98, No. 1. (Mar., 1979):
As we have seen, Jewish religion was one reason for anti-Semitism, but perhaps a more basic cause was their exclusiveness, encouraged by their law and grounded in their monotheism. To the degree that Jews in a polytheistic culture tried to maintain a strong monotheism and a unique law based on monotheism, it was inevitable that they should have trouble dealing with their neighbors…
The importance of this fact could hardly be overstated. Jewish faith bred an anti-social clannishness which in turn cast suspicion on them as citizens. They were perceived as snobbish, and in a particularly dangerous way. Since they could not and would not participate fully in Graeco-Roman political and economic life, and since they tended to maintain a strong unity among themselves, they were a force to be taken into account by all concerned…
That many Jews saw themselves as the people specially chosen by God, and therefore a better and purer race, hardly needs to be demonstrated…
Jewish authors, both of Palestine and of the Diaspora, often voice severe contempt for gentiles, even to the point of claiming that the world was created on behalf of the Jews (As. Mos. 1: 12; 4 Ezra 6:55), and that the pagan nations are “like spittle” (4 Ezra 6:56-57). Jews did not claim moral perfection for themselves, but they considered gentiles as sinners in a special sense (cf. Gal 2:15). This dichotomy in Jewish thought between the “chosen race” and the “gentile sinners” was bitterly resented by gentiles; Origen, e.g., indicates that this was one of Celsus’ primary complaints, and reiterates it several times (c. Cels. 5.41-42, 50).
The list of anti-gentile passages in Jewish literature could be made much longer… By no means did all Jews share the attitude of superiority, but a sufficiently large number did so that it occasions no surprise for gentiles to respond in kind.
No attempt can be made here to assess the degree to which the Jewish attitude merely reflected similar gentile expressions. Regardless of one’s conclusions as to the ultimate fault, it is evident that Jewish exclusiveness became a major factor in the poor relations between the races. The important thing is that this attitude was an integral part of the Hebrew religion, not mere social snobbery. The Jews’ feeling that they and they alone possessed truth, that they alone worshipped the true God and were particularly beloved by him, is the root of their exclusiveness. There were doubtless other contributing factors, but one of the most telling is to be found in the very essence of their faith. [pp. 58-60.]