Paul writes to the church at Galatia giving them some of the most stinging words in all of his letters.
Gal 1:9-10
As we said before, and now I say again, If anyone preaches a gospel to you beside what you have received, let him be accursed. For now do I persuade men, or God? Or do I seek to please men? For if I yet pleased men, I would not be a servant of Christ.

He makes it perfectly clear how he feels about someone preaching another gospel. He says let them be eternally condemned. Phew!

He also writes to the church there giving his credentials to be called an apostle. He defends his position in several of the other letters as well. He wanted people to know that his revelations from God didn’t stop at his “flash bulb” experience on the road to Damascus. He let them know that he spent other time with Jesus. He also relates how he persecuted (tried to kill) the church.
He says that he was "set apart from birth." Our understanding of the English word "birth" is that it means when a child is born and makes the entrance into the world. This is not the definition of the word "birth" in the Greek or Hebrew. The origin of a living being is at conception. So Paul acknowledges that he was "set apart" at his conception.

Peter, James (Jesus' brother, not the apostle), and John were appointed to go to the Jews and Paul to the Gentiles.

“When Peter came to Antioch, I opposed him to his face, because he was clearly in the wrong.”
Paul didn’t beat around the bush!

“What is the purpose of the law?”
The law was given as an illustration that a sacrifice must be paid for sin. Sacrifice of the animals was a foreshadowing of the sacrifice that Jesus would make for the remission of sins. The provisions of the old law were never enough. Sacrifices had to be continually made. Complete remission of sins was never satisfied. That can only come through Jesus alone. We don’t receive eternal life through strict adherence to the law. Until Jesus came people were held in bondage to the law.

“Now that faith has come, we are no longer under the supervision of the law.”
Now that Jesus has come, we aren’t bound to the strict observance of all the laws. Being given an understanding of all the laws give us better insight into the good news of salvation through Jesus, and not salvation through the law.

Some more stinging words from Paul are these:
"You were running a good race. Who cut in on you and kept you from obeying the truth?"
This is truly something to contemplate. What are some things that can "cut in" on us? When we know the truth, are we sometimes reluctant to obey the truth for fear of what others might think?

Paul speaks about an “illness” when he first preached the gospel and that it was a trial to them.
“…you would have torn out your eyes and given them to me…”

Is Paul’s vision not completely restored? Did the blinding light on that road to Damascus still have lingering affects? Did the beatings and stoning have residual affects? Did the marks on his body include a visual reminder?
“See what large letters I use as I write to you with my own hand.”

“…for I bear on my body the marks of Jesus…”
Paul was experiencing the many persecutions that he himself had inflicted on Jesus’ followers.