John Wesley on Numbers 8:7

Of purifying - Heb. of sin, that is, for the expiation of sin. This water was mixed with the ashes of a red heifer, Num 19:9, which therefore may seem to have been prescribed before, though it be mentioned after; such kind of transplacings of passages being frequent in scripture. Shave all their flesh - This external rite signified the cutting off their inordinate desire of earthly things and that singular purity of heart and life which is required in the ministers of God.