Matthew Poole and continuators on Acts 21:24

24 Them take and purifie thy self with them, and be at charges with them, that they may shave their heads: and all may know that those things whereof they were informed concerning thee, are nothing, but that thou thy self also walkest orderly, and keepest the law.

They advise Paul to perform all those Rites required of a Nazarite, the observation of which did legally purify him.

These charges were about the threefold Sacrifice which were to be offered, Numb. 6.10. two Turtles or young Pigeons, a Lamb, unleavened Bread, and Cakes of fine Flower, as Numb. 6.11, 12. and the other four spoken of, verse 23. being poor, they require Paul here to bear their charges also, that so he might appear to be the chief amongst them, and the more zealous in their Law, to take away the scandal that was taken up by the Iews against him (these Ceremonies being as yet not deadly, or evil; tho they were dead and indifferent.)

Livest according to the Law of Moses, contrary unto what they reported of thee.

Which was done at the end of their separation, and was the accomplishment of all; and they burned the Hair which they shaved off, under their Sacrifices, to shew, that all their legal performances were only acceptable unto God through Jesus Christ, who was sacrificed for us, 1 Cor. 5.7. and Heb. 10.12.

Marginal references: Numb. 6.2, 13, 18. Chap. 18.18.