The brevity of this story (Acts 21:20-27) belies its importance, both for what it tells us about Luke and for what it says about Luke's Paul. Given the anti-legalistic, anti-ethnocentric, and apocalyptic accounts of Paul, one would expect Paul to stand up to the Jerusalem leaders in much the same way that he seems to stand up to Peter in Antioch (Gal. 2). The anti-legalistic reading seems to require Paul to declare that neither Jews nor gentiles are saved by works and so Jews do not need to keep the law and no longer need to circumcise their sons. This Paul should then tell the Jerusalem leaders that he will not take part in the cultic practices that make him appear to be law observant, since doing so would be bearing false testimony. The anti-ethnocentric reading appears to require Paul to declare to the Jerusalem leaders that their insistence on law observance and their contention that ethnicity matters are idolatrous and ethnocentric. Again, this Paul should resist the call to participate in the Jerusalem cult since to do so would be to suggest that Jewish distinctives like circumcision, purity regulations, and the temple (works of the law) still matter. And the apocalyptic reading seems to require Paul to declare that the old has passed in light of the apocalyptic invasion of the Messiah: the temple, ritual purity regulations, and ethnicity are all nothing in the wake of the messianic new creation. 

But Luke's Paul does none of these things. Instead, he does precisely what is asked of him, knowing full well that it will give, and is intended to give, everyone the impression that he himself keeps the Jewish law. These publicly performed rituals show people that Paul was not teaching Jewish Messiah followers outside Judea to abandon Moses, the Jewish rite of circumcision, or the Jewish law. The story makes clear that the ethnic distinctions persist and matter in the Jesus movement: one set of practices applies to Jewish followers of Jesus, while another set of practices applies to gentile followers of Jesus (Acts 21:25). Laws, ethnicity, and differences continue to matter for Luke's Paul. - Matthew Thiessen, A Jewish Paul