Why did the Jews react so strongly when Paul said there is no difference between Jew and Gentile?
They reacted strongly because for nearly 2,000 years they had been taught they were God's special covenant people, set apart from Gentiles and not to intermarry. Paul's claim that Gentiles were just as good as Jews and that there is no difference infuriated them, so they followed and opposed him.
Les teaches that the Jewish community was sent into orbit by the idea that Gentiles are just as good as Jews. For almost 2000 years they had heard that they were special, God's covenant people, set apart from the rest of the world, not to intermarry with Gentiles, and to stay within their tribes. So when this little Jew said there is no difference, it was no wonder they reacted. Les reminds us from I and II Corinthians that Jews plagued the Apostle every step of his way. It infuriated them that he was taking the salvation of Israel's God to pagan Gentiles. They followed him from city to city and tried in any way to defeat him, plaguing him until his death. This was the crowning reason he put out that there is no difference between Jew and Gentile. Les also points to Ephesians 2: none righteous, no not one. The Pharisees, righteous in their own eyes, could have put him to death because he made it plain that no one is righteous, not even the most religious Jew.