Paul warns the Philippians to watch out for "evil workers" who "mutilate the flesh." Um, he means Christians who advocate for circumcision.
Paul says that real Christians don't need to worry about being physically circumcised. They're spiritually circumcised.
Trust him—Paul knows a thing or two about being circumcised. He was born a Jew and was circumcised when he was only eight days old.
He's descended from the tribe of Benjamin (remember him? That's Jacob's youngest son from way back in Genesis). Paul even grew up to be a Pharisee. Yup, that's those dreaded baddies from the gospels.
Paul was so into obeying Jewish law that he used his free time to persecute Christians. He kind of regrets that now.
But whatever good stuff Paul had going for him back then, he threw it all away when he came to believe in Jesus. Now, his relationship with God comes through Jesus—not from Jewish law.
Paul's main goal is to become closer to Jesus by suffering like him and even dying if need be. We'd say Paul's well on his way.
Of course, he's not there yet.
He really would like to be like Jesus, but he has to keep struggling and fighting every day to live the kind of life God wants for him. So does everyone else. Wow, what an everyman Paul is.
Paul tells the Philippians that they should try to be just like him. (Maybe minus the time behind bars.)
They shouldn't worry about what goes in their bellies (like some Christians Paul won't name here). They need to keep their focus on Jesus alone. He's the only one who can take them from ho-hum existence to awesomeness.