What did the Judaizers tell Paul's converts they had to do to be saved?
The Judaizers told Paul's converts that faith in Paul's gospel alone was insufficient for salvation. They insisted Gentiles also had to practice Judaism, submit to circumcision, and keep the law. Thus they added law-keeping and circumcision as requirements alongside the gospel.
Les explains that a Judaizer was a believer who accepted Jesus as the Christ and was saved under that kingdom gospel, but still did not comprehend the grace of God and that the law had now been satisfied and totally set aside. These Judaizers followed in Paul's footsteps and told his converts they could not be saved on Paul's gospel alone; they still had to practice Judaism, be circumcised, and keep the law. Les says this opposition just about drove the apostle up the wall and caused him to suffer as he defended his apostleship, showing the Corinthians what he had gone through for the sake of the gospel.