Why did the Jews become the chief opponents of Christianity?
They resented Gentiles embracing the Gospel and infringing upon their God, so when Paul turned to pagan Gentiles and those Gentiles were thrilled by the message, the Jews hated them and became Christianity's chief opponents. This reaction reflected human nature working contrary to God's will.
Les explains from the book of Acts that every place Paul went and preached the Gospel, when the Jews rejected it, he would go to pagan Gentiles, who would embrace it and be thrilled by it. The Jews then hated the Gentiles, because they saw them as infringing upon their God, and so the Jews became the chief opponents of Christianity. Les points out this is how human nature works contrary to the will of God: as Jews opposed Gentiles being saved, in time Gentiles began to hate Jews. Yet God intended to turn to the Gentile so that Israel would get jealous and return to where they belonged.