What background did Paul have that qualified him to be the apostle to the Gentiles?
Paul was uniquely qualified because he was a Jew's Jew, an Israelite, a Hebrew of Hebrews, of Benjamin, yet also a Roman citizen fluent in Greek and in the culture of the known world. That combination let him approach Jews as a Jew and Gentiles as a Roman.
Les explains that Paul continually defended his apostleship because of his unique calling and unique background. No one but Saul of Tarsus could have fulfilled the role of apostle of the Gentiles. He was a Jew's Jew, an Israelite, a Hebrew of the Hebrews, of the tribe of Benjamin. Yet he was also a Roman by citizenship, fluent in Greek, and fluent in the culture of the then-known world. In him culminated everything necessary to go to the Jew as a Jew and to the Gentiles as a Roman, which is why he is the apostle that he is.