Paul had his hair cut and fulfilled all the Jewish rites, but not because he changed his opinion (for that would have been a thing impure from vice), but because love is condescending.
"He declared the completion of the days of purification," that is, he himself declared about himself that the purification was completed.
Acts 21:27–29. And when the seven days were almost ended, the Jews which were of Asia, when they saw him in the temple, stirred up all the people, and laid hands on him, crying out, Men of Israel, help: this is the man, that teacheth all men every where against the people, and the law, and this place: and further brought Greeks also into the temple, and hath polluted this holy place. For they had seen before with him in the city Trophimus an Ephesian, and supposed that Paul had brought him into the temple.
Notice God's arrangement: after some were dissuaded, others, out of fear that the rest might also be dissuaded, attack Paul. Their restless character is visible everywhere.
Acts 21:30–34. The whole city was set in motion, and a concourse of the people took place; and having seized Paul, they dragged him out of the temple, and immediately the doors were shut. And when they were seeking to kill him, word came up to the commander of the cohort that all Jerusalem was in an uproar. He, immediately taking soldiers and centurions, rushed down upon them; and when they saw the commander and the soldiers, they stopped beating Paul. Then the commander, having come near, took hold of him and commanded him to be bound with two chains, and inquired who he was and what he had done. Among the crowd some were shouting one thing and some another. And when he could learn nothing certain because of the tumult, he commanded him to be brought into the barracks.
They wanted to kill him and dragged him out of the temple, so that they could commit the murder with greater boldness.
Acts 21:35–38. And when he was on the stairs, it happened that he had to be carried by the soldiers because of the violence of the crowd, for the multitude of the people followed and cried out: Away with him! As they were about to enter the barracks, Paul said to the commander: May I say something to you? And he said: Do you know Greek? Are you not then that Egyptian who before these days stirred up a revolt and led four thousand men of assassins out into the wilderness?
They say "Away with him!" in accordance with Roman custom, since it was customary among the Jews to shout this phrase against the righteous, as they also shouted against the Lord: "Away with Him" (Luke 23:18).
"Are you not that Egyptian?" There appeared a certain Egyptian, an innovator, a rebel, a deceiver, and a sorcerer. The devil flattered himself with the hope of using this man and presenting both Christ and the apostles as participants in those crimes which were committed by the followers of this Egyptian.
Acts 21:39. And Paul said: I am a Jew, a Tarsian, a citizen of no obscure city of Cilicia; I beg you, permit me to speak to the people.
We, Christians, are the true Jews, as those who profess the true faith, because the name "Judah" means "confession." Since the commander asked whether he was that Egyptian, Paul first of all deflects him from this suspicion. And so that he would not think that he was a Jew by origin, he then declares his faith. So what then? Did he deny it? By no means! He was both a Jew and a Christian.
"Allow me to speak to the people." This is the strongest proof that he was not guilty of anything—so ready was he to defend himself and so eager was he to address a word to the Jewish people.
Acts 21:40. And when he had given him permission, Paul, standing on the stairs, motioned with his hand to the people; and when there was a deep silence, he began to speak in the Hebrew language as follows.
Much contributed to him the very place — the stairs — namely, that he spoke while standing high up and being bound. To see Paul, bound with two chains, speaking to the people — what spectacle could equal this? How he was not frightened, looking upon such a multitude of agitated people!