I had not realized that, by the time of Paul, Judaism had attracted a large Gentile following. Garry Wills writes in What Paul Meant that Paul probably had much success preaching to these “Godfearers.”
[They] were inquiring and sympathetic non-Jews welcomed in synagogues… The Romans of the first century were out on quest for spiritual knowledge… [A]mong the exotic beliefs being entertained, the Jews had, for some, a special appeal, based on their monotheism (in a polytheistic world), their purity of life, and their ancient learning. (page 64-5)
Wills cites historian Robert Tannenbaum, who points out that Judaism was “therefore a more powerful rival to Christianity in the race for the Roman world” than used to be assumed. (page 66)
And Gerd Theissen argues (notes Wills):
Christianity… offered them the possibility of acknowledging monotheism and high moral principles and at the same time attaining full religious equality without circumcision, without ritual demands, [etc.]… [T]he Christian mission was luring away the very Gentiles who were Judaism’s patrons… (page 67).
This is interesting for two reasons. First, it indicates that Christianity benefitted from the prior appeal of Judaism to a segment of Romans. Second, it reveals additional causes of tension between Jews and Christians.