John Gill 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)

Do therefore this that we say to you This is said not as commanding, but as advising; and not to what was a duty, and necessary to be done as such, but as a point of prudence:

we have four men who have a vow on them ; that is, there were four men who were of the church at Jerusalem, believers in Christ, but weak ones, who were zealous of the law, and bigots to it, and who had voluntarily vowed a vow of the Nazarites; see (Numbers 6:2) .