Acts 21:1-19 describes Paul's journey to Jerusalem. After he arrived in Jerusalem, the believers there told him that the Jews wanted to harm him and what he should do so they would not harm him (verses 20-26). Even though Paul did what the believers told him to do, the Jews tried to kill him. The Romans rescued him and gave him a chance to speak to the Jews.
The Jews in Jerusalem were following the law of Moses. Even those who were following Jesus still kept the law. Both groups thought that Paul had been telling Jews in Greece not to keep the law. But it was only the Gentiles to whom Paul was saying that.
The Jews accused Paul of bringing a Gentile man into a part of the temple into which God only allowed Jews to go. They thought that God wanted them to punish Paul by killing him. (See: [[rc://en/tw/dict/bible/kt/holy]])
The Romans thought that they needed to treat only Roman citizens justly. They could do as they desired with people who were not Roman citizens, but they had to obey the law with other Romans. Some people were born Roman citizens, and others gave money to the Roman government so they could become Roman citizens.