From Reza Aslan’s Zealot:

Regardless, by the year 57 C.E., the rumors about Paul’s teachings could no longer be ignored. And so, once again, he is summoned to Jerusalem to answer for himself.

This time, James confronts Paul directly, telling him that it has come to his attention that Paul has been teaching believers “to forsake Moses” and “not circumcise their children or observe the customs [of the law]” (Acts 21: 21). Paul does not respond to the accusation, though this is exactly what he has been teaching. He has even gone so far as to say that those who let themselves be circumcised will have “cut themselves off from Christ” (Galatians 5: 2– 4).

To clear up matters once and for all, James forces Paul to take part with four other men in a strict purification ritual in the Temple— the same Temple that Paul believes has been replaced by the blood of Jesus— so that “all will know there is nothing to the rumors said about you, and that you observe and guard the law ” (Acts 21: 24). Paul obeys; he seems to have no choice in the matter.

…

After the embarrassing spectacle at the Temple, in which he was forced to renounce everything he had been preaching for two decades, Paul wanted to get as far as he could from Jerusalem and the ever-tightening noose of control placed around his neck by James and the apostles. Besides, Rome seemed the perfect place for Paul.

And on the purification ritual:

What Luke is describing in this passage is called a “Nazirite vow” (Numbers 6: 2). Nazirites were strict devotees of the Law of Moses who pledged to abstain from wine and refused to shave their hair or come near a corpse for a set period of time, either as an act of piety or in return for the fulfillment of a wish , such as a healthy child or a safe journey (James himself may have been a Nazirite, as the description of those who take the vow perfectly matches the descriptions of him in the ancient chronicles ). Considering Paul’s views on the Law of Moses and the Temple of Jerusalem, his forced participation in such a ritual would have been hugely embarrassing for him. The entire purpose of the rite was to demonstrate to the Jerusalem assembly that he no longer believed what he had been preaching for nearly a decade. There is no other way to read Paul’s participation in the Nazirite vow except as a solemn renunciation of his ministry and a public declaration of James’s authority over him— all the more reason to doubt Luke’s depiction of Paul as simply going along with the ritual without comment or complaint.