What does Acts 21:23 mean?
Before Paul followed Jesus, he persecuted the church. He chased Christians down, arresting them, and voting for their execution (Acts 26:10). After Paul met Jesus in Damascus and returned to Jerusalem, the church didn't believe news about his conversion. Barnabas had to take the chance to determine if he had really changed. Then he went all over Jerusalem, preaching about Jesus so aggressively he got death threats. So the church elders sent him home to Tarsus (Acts 9:26–30).Years later, Paul and Barnabas came to Jerusalem to confront James about the Pharisaical Christians he sent to Syrian Antioch and Galatia who kept telling the Gentile Christians they needed to be circumcised (Acts 15:1–4; Galatians 2:12). James disavowed the messengers, saying he never sent them, and agreed Gentiles did not have to convert to Judaism to follow the Jewish Messiah (Acts 15:6–29).
Given all this, it's understandable that the elders of the church in Jerusalem have mixed feelings about Paul's presence. They're happy his mission to build up churches and bring Jews and Gentiles to Christ is so successful (Acts 21:19–20). But the Pharisee-Christians are spreading a new rumor: that Paul is telling not just Gentiles, but Jews as well, to abandon the Mosaic law (Acts 21:21).
The solution the elders come up with is for Paul to take part in a very public Jewish religious rite. Four men have reached the end of their Nazirite vow, but they each need a significant sacrifice to finish. If Paul provides the animals they need, he will prove he is still dedicated to a proper understanding of the Law.