Albert Barnes Commentary Acts 21:24

"these take, and purify thyself with them, and be at charges for them, that they may shave their heads: and all shall know that there is no truth in the things whereof they have been informed concerning thee; but that thou thyself also walkest orderly, keeping the law." — Acts 21:24 (ASV)

Them take. Take with you. Join yourself with them.

And purify thyself with them. Join them in observing the forms of purification prescribed by the law of Moses in the observance of the vow of the Nazarite. The purifying here refers to the vows of sanctity which the Nazarites were to observe. They were to abstain from wine and strong drink; they were to eat no grapes, moist or dried; they were to come near no dead body, nor to make themselves "unclean"—for their father, mother, brother, or sister, when they died (Numbers 6:3–7); and they were to present an offering when the days of the vow were completed (Numbers 6:13–21).

And be at charges with them. Share with them the expense of the sacrifices and offerings required when the vow is completed. Those offerings were a ram of a year old for a burnt-offering, a sheep of the same age for a sin-offering, a ram for a thank-offering, a basket of unleavened cakes, and a libation of wine. See Numbers 6:13-20.

That they may shave their heads. The shaving of the head, or the cutting of the hair which had been allowed to grow during the duration of the vow (Numbers 6:5), was an observance indicating that the vow had been performed. Paul was requested to join with them in the expense of the sacrifices and offerings, so that, once all the ceremonies had been observed, their heads might be shaved as an indication that every part of the vow had been complied with.

And all may know. By the fact of your observance of one of the rites of the Mosaic religion, all may have evidence that it is not your purpose or practice to speak contemptuously of those rites, or to undervalue the authority of Moses.

Are nothing. Are untrue, or without any foundation.

Walkest orderly. That you live in accordance with the real requirements of the law of Moses. To walk, in the Scriptures, often denotes to live, to act, to conduct, in a certain manner. All, probably, that they wished Paul to show by this was, that he was not an enemy of Moses. Those who gave this counsel were Christians, and they could not wish him to do anything which would imply that he was not a Christian.