James and the elders were trying to solve a problem. They rejoiced in Paul's work, but a rumor about him had spread among the Jewish believers, and Paul's arrival would not stay secret. Barnes reads the question as "What is to be done?" and explains "the multitude must needs come together" as "There will be inevitably a tumultuous assemblage." Gill puts their concern this way: "there is danger in the case, the consequence may be bad; and therefore something must be done." Barnes is clear that the rumor itself was false: "this report was incorrect and slanderous."

The elders answered their own question with a plan: "Do therefore this that we say to thee: We have four men which have a vow on them" (Acts 21:23). Paul was to purify himself with them and pay their costs, so that "all may know that those things, whereof they were informed concerning thee, are nothing" (Acts 21:24). They also affirmed that Gentile believers were not bound to such things (Acts 21:25). "Then Paul took the men, and the next day purifying himself with them entered into the temple" (Acts 21:26). Near the end of the seven days, Jews from Asia saw him there and stirred up the people (Acts 21:27).

Barnes explains that from "the mention of shaving the head" in Acts 21:24, "it is evident that the vow which they had taken was that of the Nazarite," a special vow of separation to God described in Numbers 6. To "be at charges with them" (Acts 21:24) means to pay their expenses, as the ESV says, including the offerings at the end of the vow (Acts 21:26).

The rumor was that Paul taught Jews "to forsake Moses" and not "to walk after the customs" (Acts 21:21). Barnes explains "customs" as "the rites of the Mosaic economy." The elders' words about the Gentiles refer back to the letter of the Jerusalem council (Acts 15:19-20).

This question comes from leaders trying to keep a young church together. Jewish and Gentile believers were learning to live as one people, and misunderstanding threatened that. The elders first "glorified the Lord" for what God was doing among the Gentiles (Acts 21:20), and then looked for a way to quiet false reports without placing the law on Gentile believers (Acts 21:25).

Commentators weigh the plan differently. Matthew Henry notes, "Some have blamed this occasional conformity of Paul's," while "Others think the advice was prudent and good, and Paul's following it was justifiable enough, as the case stood." Barnes connects Paul's choice with his own words in 1 Corinthians 9:20: "unto the Jews I became as a Jew, that I might gain the Jews." The plan did not prevent the uproar that followed (Acts 21:27-30), yet God used even Paul's arrest to send his testimony on to Rome (Acts 23:11).