Can you please explain the vow Paul took at Cenchrea that made him cut off his hair recorded in Acts 18:18?
Paul stated once, "For though I am free from all men, I have made myself a servant to all, that I might win the more; and to the Jews I became as a Jew, that I might win Jews; to those who are under the law, as under the law, that I might win those who are under the law; to those who are without law, as without law (not being without law toward God, but under law toward Christ), that I might win those who are without law; to the weak I became as weak, that I might win the weak. I have become all things to all men, that I might by all means save some" (I Corinthians 9:19-22). Even though Paul strongly taught that Christians are no longer under the Old Testament covenant, Paul wasn't against keeping the customs of that Old Covenant so long as they did not interfere with his keeping the law of Christ. Thus when Timothy, who was Jewish because of his mother's ancestry though he had never been circumcised, traveled with Paul to Jerusalem, Paul had him circumcised, so that Timothy might be able to work among the Jews of that area. "Then he came to Derbe and Lystra. And behold, a certain disciple was there, named Timothy, the son of a certain Jewish woman who believed, but his father was Greek. He was well spoken of by the brethren who were at Lystra and Iconium. Paul wanted to have him go on with him. And he took him and circumcised him because of the Jews who were in that region, for they all knew that his father was Greek" (Acts 16:1-3). The circumcision of Timothy had nothing to do with the keeping of the Old Testament, it was a means of allowing Timothy to fit in so he might teach among those who firmly held that circumcision was essential.
"So Paul still remained a good while. Then he took leave of the brethren and sailed for Syria, and Priscilla and Aquila were with him. He had his hair cut off at Cenchrea, for he had taken a vow" (Acts 18:18). The shaving of the head in connection with a vow is following the custom of the Nazirite vow (Numbers 6:2-21). The Bible doesn't record why he chose to follow the Jewish practice for this particular vow, though we can make a reasonable guess. He stated he was heading to Jerusalem and wanted to be there in time for a particular feast (Acts 18:21). Paul was well known in the Jewish circles as one who had abandoned their faith and had become a Christian. By following the custom of this vow, Paul was showing everyone that he had not changed that much and it would provide an opportunity for him to talk to his brethren about the Christ. It was not that Paul thought that shaving the head was required for a vow. He saw it as something that made no difference and so it would do no harm to follow the custom and it would open doors of opportunity that he would not have otherwise. Even though he kept one part of the Nazirite vow, he would not have followed this type of vow completely. Along with the shaving of his head, the vow required abstaining from all grape products. As a Christian, Paul would not have done this if the vow lasted longer than a week because he would partake of the Lord's Supper on the first day of the week (Acts 20:7); that supper includes the fruit of the vine (grape juice).
He was later encouraged to do something similar for the same purpose. "And when they heard it, they glorified the Lord. And they said to him, "You see, brother, how many myriads of Jews there are who have believed, and they are all zealous for the law; but they have been informed about you that you teach all the Jews who are among the Gentiles to forsake Moses, saying that they ought not to circumcise their children nor to walk according to the customs. What then? The assembly must certainly meet, for they will hear that you have come. Therefore do what we tell you: We have four men who have taken a vow. Take them and be purified with them, and pay their expenses so that they may shave their heads, and that all may know that those things of which they were informed concerning you are nothing, but that you yourself also walk orderly and keep the law. But concerning the Gentiles who believe, we have written and decided that they should observe no such thing, except that they should keep themselves from things offered to idols, from blood, from things strangled, and from sexual immorality." Then Paul took the men, and the next day, having been purified with them, entered the temple to announce the expiration of the days of purification, at which time an offering should be made for each one of them" (Acts 21:20-26). Notice that the suggestion for observing the vow was not to bind the Old Law. It was clearly stated, "But concerning the Gentiles who believe, we have written and decided they should observe no such thing ..." This was merely a way for Paul to demonstrate that he wasn't the Jews' enemy. Following this practice did not break Christian principles, so long was it was not taught as being binding on all Christians (which it was not), so Paul used it as a means of gaining the acceptance of the Jews so that he could teach them.
"For though I am free from all men, I have made myself a servant to all, that I might win the more; and to the Jews I became as a Jew, that I might win Jews; to those who are under the law, as under the law, that I might win those who are under the law; to those who are without law, as without law (not being without law toward God, but under law toward Christ), that I might win those who are without law; to the weak I became as weak, that I might win the weak. I have become all things to all men, that I might by all means save some" (I Corinthians 9:19-22).
It didn't work in this case, but I want you to notice the charge that was laid against Paul: "Men of Israel, help! This is the man who teaches all men everywhere against the people, the law, and this place; and furthermore he also brought Greeks into the temple and has defiled this holy place" (Acts 21:28). Though Paul did not break the law as they had charged, still people knew that Paul taught against following the law; that is why it was so easy to get the crowd to imagine that he had broke the law.