on Acts 21 :31
The chief captain of the band - The Roman tribune, who had a troop of soldiers under him, which lodged in general in the castle of Antonia, which was built at the angle where the northern and western porticoes of the outer court of the temple were joined together. This castle was built by John Hyrcanus, high priest of the Jews: it was at first called Baris, and was the royal residence of the Asmoneans, as long as they reigned in Jerusalem. It was beautified by Herod the Great, and called Antonia, in honor of his friend Mark Antony. By this castle the temple was commanded, as it stood on higher ground. Josephus describes this castle, War, b. v. chap. 5, sec. 8, "as having four towers, from one of which the whole temple was overlooked; and that one of the towers was joined to the porticoes of the temple, and had a double pair of stairs from it, by which soldiers in the garrison were used to come down with their arms to the porticoes, on the festival days, to keep the people quiet; for, as the temple was a guard to the city, so this castle was a guard to the temple." "It seems, therefore," says Bp. Pearce, "to me very plain, that the place where the Jews were about to kill Paul was the court of the Gentiles, the porticoes being there; and that the chief captain came down there to his rescue." The name of this chief captain, or tribune, was Claudius Lysias, as we learn from Acts 23:26.
on Acts 21 :31
And as they went about to kill him - Greek: they seeking to kill him. This was evidently done in a popular tumult, as had been done in the case of Stephen, Acts 7:They could not pretend that they had a right to do it by law.
Tidings came - The news, or rumour came; he was told of it.
The chief captain of the band - This band or body of Roman soldiers was stationed in the castle Antonia, on the north of the temple. This was built by John Hyrcanus, high priest of the Jews, and was by him called Baris. It was beautified and strengthened by Herod the Great, and was called Antonia in honor of his friend, Mark Antony. Josephus describes this castle as consisting of four towers, one of which overlooked the temple, and which he says was 70 cubits high (Jewish Wars, book 5, chapter 5, section 8). In this castle a guard of Roman soldiers was stationed to secure the temple and to maintain the peace. The commander of this cohort is here called "the chief captain." Reference is made to this guard several times in the New Testament, Matthew 27:65-66; John 18:12; Acts 5:26. The word translated "chief captain" denotes properly "one who commanded 1,000 men." The band σπεῖρα speira was the tenth part of a legion, and consisted sometimes of four hundred and twenty-five soldiers, at others of five hundred, and at others of six hundred, according to the size of the legion. The name of this captain was Claudius Lysias, Acts 23:26.
In an uproar - That the whole city was in commotion.
on Acts 21 :31
21:31 And as they went about to kill him - It was a rule among the Jews, that any uncircumcised person who came into the inner temple, might be stoned without farther process. And they seemed to think Paul, who brought such in thither, deserved no better treatment. Word came to the tribune - A cohort or detachment of soldiers, belonging to the Roman legion, which lodged in the adjacent castle of Antonia, were stationed on feast days near the temple, to prevent disorders. It is evident, Lysias himself was not present, when the tumult began. Probably he was the oldest Roman tribune (or colonel) then at Jerusalem. And as such he was the commanding officer of the legion quartered at the castle.