Who were Paul's opponents when he preached the gospel of grace in Acts 15?
Paul’s opponents in Acts 15 were Judaizers—Jewish believers still under the law. Certain men from Judea came to Antioch and taught Gentile believers that unless they were circumcised after the manner of Moses, they could not be saved. These law-bound teachers opposed the gospel of grace.
Les explains that Acts 15 is the same set of events Paul reports in Galatians 2. The conflict arose when certain men came down from Judea and taught the Gentile believers at Antioch, “Except you be circumcised after the manner of Moses, you cannot be saved.” These Judaizers, still under the law, were what Paul was up against as he tried to bring the gospel of the grace of God. The Jerusalem believers were so saturated with the law that they tried to bring the Gentile believers of Paul’s ministry under it.