A proper cause for boasting
Like the rest of us, Paul had plenty of opportunities for boasting if he were inclined to seek them. He lists some of them in Philippians 3:5-6. Circumcised. Descended from Abraham, Isaac and Jacob. Devout follower of Jehovah God for his entire life. A spotless reputation for righteousness among all who knew him. Such things were, in fact, used by many Jewish Christians to distance themselves morally and spiritually from others, particularly Gentiles.
But Paul abandoned such things.