Paul’s opponents may have claimed to be better qualified to explain the role of the Law for Gentiles because of their heritage and training. This is more or less equivalent to someone who claims to be an expert because they graduated with a PhD from Harvard as opposed to a certificate from DeVry Institute (Fill-in your own institutions here.) Paul therefore takes a moment to boast about his personal heritage and achievements. Paul claims in Philippians is that he is a proper Jew who excelled in the practice of Second Temple Judaism more than anyone else of his generation. To use a phrase from the Harry Potter universe, Paul is claiming to be a proper Jew rather than some Mudblood.
First, he was circumcised on the eighth day. This indicates he comes from a family that is keeping the Jewish traditions despite living in Tarsus. It is possible that there were Diaspora Jews who did not keep this tradition or even did not circumcise their boys.
Second, he is a member of Israel. This connects Paul to the covenant as a member of Abraham’s family. Paul was not a Hellenistic Jew from Tarsus pretending to be a Greek, but rather a Jew who was well aware of his heritage as a child of Abraham.
Third, he is from the tribe of Benjamin. This is significant because not every Jew in the first century could claim to know they were from a particular tribe. Paul’s Jewish name “Saul” is taken from the first king of Israel, from the tribe of Benjamin, and Paul’s teacher in Jerusalem, Gamaliel, was also from the tribe of Benjamin.
Fourth, the phrase “Hebrew of the Hebrews” can be taken in several ways. This phrase may mean that Paul was born of true Jewish blood, that there is no Gentile in his linage. It is sometimes suggested that Paul is referring to his ability to speak and read Hebrew. Not all Jews spoke the language, especially in the home. If there is an increasing specificity in the list of descriptions, then perhaps Polhill is right and Paul is saying he is from an extremely Jewish family, one that still speaks the language at home (Paul and his Letters, 26).
Fifth, with respect to religion, he is a Pharisee. Second Temple Judaism had a number of sub-divisions, not exactly like modern denominations but that is a fair way to think about them. Sadducees and Pharisees are the two most well known in the New Testament, but there were several others. There are many ways to define the groups, but Paul’s emphasis might be on faithfulness to the Law and loyalty to Israel. Pharisees were not simply observant of the Law; they thought deeply about the Law and guarded themselves against breaking the Law unknowingly. They really could claim to be “blameless” with respect to law
Last, with respect to zeal, Paul says he was a persecutor of the church. Zeal has become a Christian virtue in modern Church-talk, usually equivalent to strong emotional response in worship. But that is not at all Paul’s point here. He is zealous for the Law and the traditions of his people in the same way the Maccabean Revolt was zealous for the Law. In that case, they fought the Greeks for the right to keep the Law. More important, they were willing to enforce the Law for Jews, including circumcision. Paul’s zeal was not a warm feeling of love for God, he was violently opposed to Jews who claimed the Messiah was crucified by the Temple authorities; he was willing to use physical abuse to convince people this Messiah did not rise from the dead.
Paul is, in the words of J. B. Lightfoot, making a progressive argument. A convert to Judaism may be circumcised, someone with some Gentile in his linage might claim a tribal affiliation, but Paul is a pure-bred true Jew! If anyone in the Second Temple period could boast about their heritage it was Paul.
These achievements are important since they contribute to who Paul is in Christ. His education and training in the Hebrew Bible are the foundation for this evangelism and all of his writings. He is just a zealous serving Christ as he was pursuing the church of God. Yet (like Jesus) he sets these achievements aside in order to be called a servant of God.