Was the Apostle Paul angry with the Galatian Church?
This letter isn't about Paul's ego or preferences: it's about understanding why Jesus had to die and how it affects us.
The Jews had been living under the Law since the days of Moses. The Law was a set of expectations for God's people: commands that, when followed, would distinguish Israel from all other nations as a people that belonged to God. However, Israel couldn't keep the Law. Nobody could: everyone was a sinner.
So God sent Jesus. Jesus lived the Law, died for our sins, and rose again---He fulfilled the Law so we don't have to.
The Galatians' new teacher completely disregards and disrespects God's grace, Christ's sacrifice, and the Holy Spirit's work. That's why Paul is so upset.
This book explains the believer's new relationship with God. We're freed from sin. We're freed from the Law. We're adopted as children of God. We're counted as spiritual children of Abraham, whether we're Jews or non-Jews. And we're all empowered by the Holy Spirit to do good works, something sin prevented us from doing and the Law never enabled us to do.
Christ's death is important, and Paul won't let anyone forget it.