Acts  21:21-25
Acts 21:21 (KJB)
And they are informed of thee, that thou teachest all the Jews which are among the Gentiles to forsake Moses, saying that they ought not to circumcise their children, neither to walk after the customs.
False teachers had made their way among the Jewish population and continued to level these false accusations against Paul. They taught that one must become Jewish in faith and then one can believe in Christ. When you continue to make accusations, people begin to start believing them and this is why Paul was going up against a very biased population because the seeds of lies had been planted in both the people and the leaders.
Acts 21:22 (KJB)
What is it therefore? the multitude must needs come together: for they will hear that thou art come.
Now that word will spread that Paul is in Jerusalem, the people will come together to want to hear exactly what Paul had been teaching. Now they will be suspicious of Paul because of all the previous lies they have been told. There were thousands in Jerusalem that were Christians but they were still faithful to the law and these were the ones who had misunderstood Paulís ministry and his motivations. They had to have a controlled crowd or else there could be a riot, remember Ephesus, so they had to come up with something whereby they could show that Paul was not the false teacher he is accused of being.
Acts 21:23 (KJB)
Do therefore this that we say to thee: We have four men which have a vow on them;
They then suggested to Paul that there is four men which have a vow upon them. The way the verse is worded, these four men may have been from the Jerusalem church but we are not sure. It was probably a Nazarite vow which showed a personís dedication to God and His will. They were not to shave their head, nor drink wine or grape juice, nor touch a dead body and by these things they showed that their love for God was higher than love for humanity. At the end of the time of the vow, an expensive sacrifice was offered and then they would shave their heads.
Acts 21:24 (KJB)
Them take, and purify thyself with them, and be at charges with them, that they may shave their heads: and all may know that those things, whereof they were informed concerning thee, are nothing; but that thou thyself also walkest orderly, and keepest the law.
This was being done to prevent riots from starting and to show that Paul was not the enemy of the Jewís religion. Paul did not have to take the vow himself, but was asked by the Jerusalem church leaders to go through the ceremony of purifying himself with the four men. This may also appease those believing Jews that Paul was purifying himself because he stayed with Gentiles and ate food which was not under the dietary laws of the Mosaic law, namely kosher. He also paid the expenses of the four men he was with which would show the whole assembly that Paul did not teach anyone to go against the customs of the fathers. The expenses would have been associated with the sacrifice at the end of the vow. These things would show the people in Jerusalem that Paul did not teach against Moses and that he was not an enemy of the law.
Acts 21:25 (KJB)
As touching the Gentiles which believe, we have written and concluded that they observe no such thing, save only that they keep themselves from things offered to idols, and from blood, and from strangled, and from fornication.
James and the elders had now confirmed the results of the Jerusalem council seven years ago. The Gentile believers were not commanded to be circumcised nor to keep the law of Moses. The Jerusalem church was still standing by that council and had not changed any of its understandings nor did they add any of the requirements set forth by the Judaizers. They understood that these things concluded in the council were prohibitions to keep the believer from going back into the old lifestyle, especially those that came out of idolatry. The elders in the Jerusalem church did not want Paul to think that they were part of that mindset about keeping the law, and even asking Paul to purify himself which was done for the purpose of preventing a riot.