John Calvin Commentary Acts 21:31

"And as they were seeking to kill him, tidings came up to the chief captain of the band, that all Jerusalem was in confusion." — Acts 21:31 (ASV)

As they sought to kill him. Assuredly, the force of Satan is apparent in this: he drives the people headlong into such rage that, when they have shut the doors of the temple, not being content with lesser punishment, they conspire to put Paul to death. We must therefore reflect that Satan urges on the enemies of godliness, lest their rage, however cruel and troublesome it may be, trouble us.

On the other hand, the wonderful goodness of God appears when He suddenly raises up the chief captain to deliver Paul from death. He himself had no such intention; he came only to appease the tumult that had arisen among the people. But the Lord shows a more evident sign of His providence in this, that Paul’s life was delivered from such immediate danger without human counsel. Thus He allows the faithful not only to labor but also to be almost oppressed, so that He may deliver them from death all the more wonderfully. Luke calls him the chief captain [tribune] of the band improperly, since every such chief captain (a tribune) was set over a thousand men. This also appears from the text, where it says that the chief captain took subordinate captains with him.