Why was it so hard for Jews of Paul's day to accept that there is no difference between Jew and Gentile?
Jewish tradition held that God had set Israel apart from Egypt onward, commanding separation from the pagans and forbidding intermarriage. That ingrained identity of being different made Paul's claim of no difference between Jew and Gentile offensive; it contradicted their long-standing tradition and national distinctiveness, so it riled them.
Les explains from Romans 3 beginning at verse 21 that the righteousness of God without the law is manifested, witnessed by the law and the prophets, and is by faith of Jesus Christ unto all and upon all who believe, plus nothing. Then comes the statement “there is no difference,” meaning no difference between Jew and Gentile. Les says this is what got Paul in trouble with his Jewish people, because he maintained there was now no difference. The difficulty went back to Exodus: as soon as God pulled Israel out of Egypt, He told them they were different, that He would set them apart. He instructed them not to have anything to do with the pagans around them and not to intermarry with them. Les teaches that it was hard for the Jews of Paul's day to suddenly come out of that tradition that they were different and hear this Jewish man say there is no difference. That is why it riled them. The case remains, however: there is no difference, because all have sinned, not just the Gentiles.