Acts 21:17-26 (LSB)
17 And after we arrived in Jerusalem, the brothers welcomed us gladly.
18 And the following day Paul went in with us to James, and all the elders were present.
19 And after he had greeted them, he began to relate one by one the things which God did among the Gentiles through his ministry.
20 And when they heard it they began glorifying God; and they said to him, “You see, brother, how many thousands there are among the Jews of those who have believed, and they are all zealous for the Law;
21 and they have been told about you, that you are teaching all the Jews who are among the Gentiles to forsake Moses, telling them not to circumcise their children nor to walk according to the customs.
22 What, then, is to be done? They will certainly hear that you have come.
23 Therefore do this that we tell you. We have four men who are under a vow;
24 take them and purify yourself along with them, and pay their expenses so that they may shave their heads. Then all will know that there is nothing to the things which they have been told about you, but that you yourself also walk orderly, keeping the Law.
25 But concerning the Gentiles who have believed, we wrote, having decided that they should keep from meat sacrificed to idols and from blood and from what is strangled and from sexual immorality.”
26 Then Paul took the men, and the next day, purifying himself along with them, went into the temple giving notice of the completion of the days of purification, until the sacrifice was offered for each one of them.
Extra Scripture: James 1:26, 1 Corinthians 9:19-23
"These were Jewish Christians who remained devoted to the ceremonial aspects of the law. While not viewing it as a means of salvation, they still observed its required feasts, Sabbath regulations, ritual vows and dietary restrictions." (Dr. John MacArthur)
"Paul's arrival in Jerusalem probably was in Spring of A.D. 56 or 57 during the procuratorship of Felix. Josephus described this period of the mid-50's as a time of intense Jewish nationalism and political unrest. One insurrection after another rose to challenge the Roman overlords, and Felix brutally suppressed them all. This only increased the Jewish hatred for Rome and inflamed anti-Gentile sentiments. It was a time when pro-Jewish sentiment was at its height, and friendliness with outsiders was viewed askance. Considering public relations, Paul's mission to the Gentiles would not have been well received." (Dr. J.B. Polhill)