What does Acts 21:20 mean?
Paul and the elders of the church in Jerusalem have very different lives. Paul travels a great deal, sharing Jesus' story to Gentiles in pagan towns who know little or nothing of the Jewish God. The elders live in Jerusalem, trying to teach about Jesus to Jews, including Pharisees who think they know everything about the Jewish God. Paul calls Gentiles from their pagan lives to a new life in Christ. The elders call out Pharisees who want this Jesus but don't want to sacrifice their esteemed place in the community provided by rigid law-keeping.This came to a head after Paul and Barnabas returned from their first missionary journey. Pharisaical Christians from Jerusalem told Gentiles they needed to follow the Mosaic law—by being circumcised and keeping a kosher diet—to properly worship the Jewish Messiah. Paul and Barnabas fiercely disagreed and brought the issue to the elders and apostles in Jerusalem. James, the pastor, disavowed the Pharisees and their message and wrote a letter telling Gentiles to refrain from sexual immorality and eating blood and food sacrificed to idols (Acts 15). Paul willingly shared the letter with the churches he planted.
Years later, while Paul is working around the area of the Aegean Sea, a rumor spreads. This is something of the inverse of the prior error: that Paul is not only failing to ask Gentiles to follow the council's reasonable requests, but he's also telling Jewish Christians they should no longer follow the Mosaic law (Acts 21:21).
When Paul denies the accusations, the church in Jerusalem is relieved. Welcoming Gentiles into the church is one thing; completely abandoning the symbol of Jewish national identity is quite another. God's commands to the nation of Israel don't contradict the gospel of salvation. It would have been difficult for evangelists in Jerusalem to witness to Jewish Pharisees if Paul were travelling the world telling Jewish Christians not to be circumcised.
Their solution is for Paul to help a group of men perform the final rites of their Nazirite vow. Ironically, when Paul does so, Jews from a port city on the Aegean Sea accuse him of bringing a Gentile into the synagogue. They start a riot. Paul is "rescued" when the Roman guards arrest him (Acts 21:22–36).