circumcised on the eighth day, of the people of Israel, of the tribe of Benjamin, a Hebrew of Hebrews; as to the law, a Pharisee; (Philippians 3:5, ESV)Paul wasn't just circumcised... he was circumcised on the eighth day, just like the Law prescribes. He is also a Jew by birth, and he knows his lineage: he is of the tribe of Benjamin. He is "a Hebrew of Hebrews!"
The Pharisees had taken apart the Law, command by command, and sought to understand this Law. For example, if the Law says to not work on the Sabbath, the Pharisees had sought to understand what exactly work is! This many steps is work, but if you only take this many steps it isn't. Moving this many pieces of wood into the fire is OK, but if you move this many, you have entered into work.
Paul was one of these guys.
If there was confidence in the flesh to be had... he would win.