Why were the Jerusalem believers upset by Paul's ministry to the Gentiles?
The Jerusalem believers were upset because Paul claimed Gentiles were saved without keeping the law of Judaism. They considered this impossible, saw it as spying out liberty in Christ, and sought to bring Gentile believers back under the law's bondage.
Les explains that the Jerusalem believers, including Peter and the rest, were all shook up because Paul was claiming the Gentiles as saved people even though they were not keeping the law of Judaism. To them this was impossible. False brethren had been brought in secretly to spy out the liberty believers have in Christ Jesus—freedom from the law. Their aim was to bring Paul, Barnabas, and the Antioch Gentile believers into bondage, back under the law. These Jewish believers of Jerusalem were still trying to hang the law on them. Paul says they did not give in by subjection, no, not for an hour.