What does Acts 21:31 mean?

As Paul spent the last few months traveling around Greece, Macedonia, and modern-day Turkey, the Holy Spirit continually warned him he would be arrested and suffer in Jerusalem (Acts 20:22–23). This wasn't so Paul would avoid Jerusalem; it was to prepare him for what was coming. It is God's will that Paul be arrested. That experience will place Paul exactly where he needs to be to promote the gospel.

That time has come. In the process of attempting to prove he is still a Jew, Paul is falsely accused of bringing a Gentile into the temple. This is against both the Mosaic and Roman laws and is punishable by death—in fact, in 1871, a stone in a Muslim school in Jerusalem was found engraved with such a warning. The crowd of Jews grab Paul, drag him out of the temple, and shut the doors lest they spill human blood in the holy site (Acts 21:27–30).

On the northwest corner of the temple mount stands the Antonia Fortress: the Roman Empire's military garrison in Jerusalem. The base sits level with the city, but the tower rises above the temple courtyard so the guards can watch for trouble. "Tribune" can identify different positions, including those in charge of administration and logistics who are also on-call to lead units into battle. Among other things, they are to protect the poor and the non-citizens from the aristocrats. Since the governor's headquarters is in Caesarea Maritima, sixty miles away, he is also responsible for keeping the peace.

The tribune arrests Paul thinking he is the Egyptian who attempted to lead Jewish assassins in revolt against the Roman occupiers in Jerusalem (Acts 21:37–40). Seeking to find out what is going on, he, perhaps foolishly, allows Paul to explain himself to the crowd, which leads to further unrest (Acts 22:1–22). After realizing Paul is a Roman citizen (Acts 22:22–29) and the Jewish council is both clueless and corrupt (Acts 23:7–10, 12–22), he wisely decides the situation is above his paygrade and sends Paul to the governor (Acts 23:23–35).