Then they said to him, “You see, brother, how many thousands of believers there are among the Jews, and they are all zealous for the law. They have been told about you that you teach all the Jews living among the Gentiles to forsake Moses, and that you tell them not to circumcise their children or observe the customs. What then is to be done? They will certainly hear that you have come. So do what we tell you. We have four men who are under a vow. Join these men, go through the rite of purification with them, and pay for the shaving of their heads. Thus all will know that there is nothing in what they have been told about you, but that you yourself observe and guard the law. But as for the Gentiles who have become believers, we have sent a letter with our judgment that they should abstain from what has been sacrificed to idols and from blood and from what is strangled and from fornication.” Then Paul took the men, and the next day, having purified himself, he entered the temple with them, making public the completion of the days of purification when the sacrifice would be made for each of them. Acts 21:20-26
Paul and the Apostles teach that the Mosaic Law is fulfilled in Jesus Christ and not attainable by us except through His blood shed for us. It is Jesus Christ who is perfect and we are not, glory to God for Christ, and therefore we can only receive our salvation through confession and forgiveness to Jesus our Christ. Christ forgives and then takes us into His grip of obedience.
The proposal given to Paul by James here in Jerusalem, and from the Jerusalem Church, is for Paul to go through the Jewish purification rite and pay for four other men whom he joins during the same rite. Paul does this although it is not his belief. Paul takes the men, purifies himself, and enters the temple with the men to make public his actions lasting seven days.
James and Peter, long before this, have agreed that the Mosaic Law is no more, that circumcision is not required, and the customs of Law are not required. In Acts 15:10-11 Peter says, “Now therefore why are you putting God to the test by placing on the neck of the disciples a yoke that neither our ancestors nor we have been able to bear? On the contrary, we believe that we will be saved through the grace of the Lord Jesus, just as they will.” The unbearable yoke of the Law has been abolished by Christ.
When Mosaic Law purification is first proposed by James to Paul, to pacify those who are greatly troubled by what Paul is saying to the Gentiles, Paul could have said, “I do not keep the Mosaic Law and neither should all of you.” Peter does not keep the Mosaic Law. In Acts 11:3-18 God commands Peter not to keep it and to give the Gentiles Word of Salvation in Christ.
The Holy Spirit is guiding and directing Paul. Christ is with him and he is obedient as “the prisoner in the Lord.” (Ephesians 4:1) And so then, at the beginning of the accomplishment of the seven days purification rite, Paul is seized by those who seek to silence him.