After noting this famous Pauline passage has tended to be interpreted in terms of a polemic against a proto-Pelagian soteriology Alastair Roberts goes on to set this option aside in favor of interpreting the passage in light not of works but of status indicators and privilege.
... Whatever we might say about his later Torah-observance and zeal, being circumcised on the eighth day, being an Israelite, being a member of the tribe of Benjamin, and having impeccable Hebrew pedigree were largely accidents of Paul’s birth, unrelated to anything that he himself had done.
Instead of serving as signs of moral attainment, these biographical details were indicators of covenant status, signs that Paul was situated—or so he once thought—on the inside track of God’s purposes. We need not, of course, just switch from a reading focusing entirely upon performance to one that speaks only of status: both are present. However, matters come into clearer focus when we understand the sort of identity that Paul once boasted in, not least because similar genres of identities continue to exert a powerful force in our own world.
If the identity that Paul is describing here is not that of the classic legalist, what is it? I believe that an analogous sort of identity could be found in the patriot. Paul wasn’t that unlike the patriot who takes pride in the fact that he is a true American (as opposed to all of those unwelcome immigrants). ...
It gets more fun and interesting from there, with Roberts pointing out how the status indicators that Paul at one point could take great pride in he came to consider rubbish.