Paul and the other apostles are writing to the church in Galatia, which is apparently already worshiping other gospels. He curses the false preachers and says if he was trying to win a popularity contest, he'd tell them what they wanted to hear, but he's a servant of Christ, so he tells them what they need to hear.
He reminds us about how he used to persecute Christians, until god spoke to him on the road to Damascus. Or possibly he was in Arabia. Anyway, he was somewhere, and he spoke to god. Then he went to Jerusalem and spoke to Peter and James.
Paul went around hectoring people for a few years, but then had a revelation that sent him back to Jerusalem to get his new message for the Gentiles approved. They agreed and didn't make his companion Titus get circumcised, for which I'm sure Titus was eternally grateful. They had this conversation because of some false christians who wanted to kidnap them and sell them into slavery. But they were careful, because they wanted to get to evangelising. The apostles approved the message, with the proviso that they also had to remember the poor.
Later, at Antioch, he got into an argument with Peter, who refused to eat with the gentiles in public, which led other Jewish christians to act the same, so he pointed out that salvation is through Jesus, not circumcision.
Paul calls the Galatians fools, which seems like the easiest way to lose your audience. He asks if they got the word through obeying the law, or faith, and why they're so materialistic.
Following the law makes you cursed, because Jesus. The laws were just a temporary thing between Abraham and Jesus. A prison guard, or a teacher. Then he tells us we're all one under Jesus, conveniently forgetting that part in just the last book where he said women shouldn't speak in church.