Albert Barnes Commentary Acts 21:23

"Do therefore this that we say to thee: We have four men that have a vow on them;" — Acts 21:23 (ASV)

We have four men. There are four men with us. It is evident that James and the elders meant to say that these men were connected with them in the Christian church. That act shows that the Christians at Jerusalem did not disregard the institutions of Moses and had not yet been so far enlightened in the doctrines of Christianity as to forsake the ceremonial rites of the Jews.

Which have a vow on them. They have made a vow. See the comments on Acts 18:18.

From the mention of shaving the head (Acts 21:24), it is evident that the vow they had taken was that of the Nazarite. As the time of their vow was about to expire, they were about to be shaved, in accordance with the usual custom on such occasions. See the comments on Acts 18:18.

Paul could join these persons and thus show decisively that he did not intend to undervalue or disparage the laws of Moses when those laws were understood as mere ceremonial observances.