Why did the Jerusalem believers try to force the law onto Paul's Gentile believers?

The Jerusalem believers were still so saturated with the law that they could not fathom the God of Abraham saving Gentiles without bringing them under Israel's law. Judaizers from Judea taught that circumcision after the manner of Moses was necessary for salvation, so they pressed that requirement on Paul's Gentile believers at Antioch.

Les explains that the Jerusalem believers were still saturated with the law, so they could not comprehend that the God of Abraham would go to the pagans and bring them to salvation without bringing them under the law of Israel. In Acts 15, certain men came down from Judea and taught the Gentile believers at Antioch that unless they were circumcised after the manner of Moses, they could not be saved. Les ties this to Galatians 2 as a direct parallel, about 22–23 years after Pentecost. Paul was trying to bring the gospel of the grace of God while up against Judaizers who were still under the law. That is why they tried to bring Paul's Gentile believers under it.

Acts 15:1

And certain men which came down from Judaea taught the brethren, and said, Except ye be circumcised after the manner of Moses, ye cannot be saved.

Why did the Jewish believers at Jerusalem try to bring Gentile believers under the law?

They were still deeply saturated with the law of Israel and could not fathom that the God of Abraham would save Gentiles without requiring them to come under that same law. Certain men from Judea even insisted circumcision according to Moses was necessary for salvation.