Who opposed Paul's gospel to the Gentiles and what did they teach?
The Judaizers from Jerusalem opposed Paul's gospel to the Gentiles. They undermined it by trying to convince Gentile converts that they had to keep the Law of Moses, be circumcised, and do these things, thus adding law-keeping to the message of grace.
Les explains that after Saul of Tarsus was saved in Acts chapter 9 and sent to the Gentiles, everything began to change, and God made clear in Romans chapter 6 that believers are not under law but under grace. The moment Gentile converts came under His Gospel, the Judaizers from Jerusalem began to undermine it. They were still trying to convince these Gentiles that they had to keep the Law of Moses, be circumcised, and do those things. The Galatians were falling to that, which is why Paul makes a stringent statement, returning to Galatians chapter 1 for review.