What does Acts 22:18 mean?

Paul is explaining to a mob of Jews in Jerusalem who want him dead that, years before, Jesus had told him to leave Jerusalem because some Jews wanted him dead.

In the previous event, Paul had gone to Damascus to arrest and imprison Jewish Jesus-followers, but met Jesus on the way and later returned to Jerusalem a Christian himself (Acts 9:3–19). In this event, Paul has returned to Jerusalem with several Gentile believers. Some of the Jews who recognize one of the Gentiles saw Paul and his friend in Jerusalem together in the city (Acts 21:29).

In the previous event, Paul preached about Jesus so aggressively that devout Jews tried to kill him (Acts 9:29). In this event, the devout Jews who saw Paul with his friend later saw Paul in the temple and assumed he had desecrated God's house by bringing the Gentile man with him, so they try to kill him (Acts 21:27–28, 31).

In the previous event, the apostles realized Paul was in danger and sent him home to Tarsus and Jesus agreed with their decision (Acts 9:30). In this event, the Holy Spirit has already told Paul he will be arrested (Acts 20:22–23). He will spend the next five years under house arrest.

The Jews' rejection of their Messiah breaks Paul's heart. He has a passion for the Jews (Romans 9:3–5; 10:1) and all the bona fides to reach them (Acts 22:3–4; Philippians 3:4–6). In the previous event, God may have let Paul see the opposition to his teaching in Jerusalem to convince him he needed to go to the Gentiles, which He had told him when he accepted Christ (Acts 26:17–18). In this event, he will be sent to Caesarea Maritima in two days (Acts 23:31–33). There is no record that Paul ever returns to Jerusalem again. As the mob dragged him from the temple and shut the gates (Acts 21:30), so, apparently, God keeps him from the city of the Jews.