It ain’t braggin if you can back it up
The Apostle Paul has been called many things: wise, the founder of the “modern” church, the Apostle to the Gentiles. And arrogant. Yeah, let’s name it: Paul has been called pretty arrogant.
But what sounds like boasting, in his own words, isn’t boasting, but a statement of fact. And it’s not boasting because he doesn’t take credit for it: he gives the credit to Christ, who is the true author of our faith.
All things Paul says about himself seem like boasting, but it’s not boasting if you can back it up, and Paul backs up his faith by recognizing that, apart from Christ, he has nothing to boast about.