Why did the early Jewish believers have trouble accepting Gentile converts who were not under the law?
They were so saturated with the Mosaic law that they could not conceive of God saving Gentiles without first bringing them under Israel's law. They believed circumcision and law observance were necessary for salvation, as Judaizers from Judea insisted at Antioch. Jewish believers in Jerusalem tried to bring Paul's Gentile converts under that law.
Les explains that the early Jewish believers' difficulty was not mere prejudice but a deep saturation with the law. They could not fathom that the God of Abraham would go to those pagans and bring them to salvation without bringing them under the law of Israel. In Acts 15, a direct parallel to Galatians 2, certain men from Judea came to Antioch and taught the Gentile believers that unless they were circumcised after the manner of Moses, they could not be saved. The Jerusalem Jewish believers were still so saturated with the law that they tried to bring Paul's Gentile converts under it. This was the opposition Paul faced as he brought the gospel of the grace of God amid Judaizers who were still under the law.