Numbers 19:21

And it shall be a perpetual statute unto them,.... To the children of Israel, throughout their generations, unto the coming of the Messiah, when the ceremonial law, which stood in divers washings and purifications, was abolished:

that he that sprinkleth the water of separation shall wash his clothes; the priest that sprinkled, according to the Targum of Jonathan, or any other person that did it; so that the same purifying water, which made an unclean person clean, defiled a clean one; for though it was purifying, it had uncleanness in it; having the ashes not only of the cow itself, but of its skin, blood, and dung; and so a lye made of ashes is impure in itself, and yet serves to scour cloth: Ainsworth thinks this signifies the imperfection and insufficiency of legal rites, which, in their greatest virtue, only sanctified to the purifying of the flesh, and left the purifier himself in uncleanness he had not before; by consideration of which, the people might be led to Christ, and his Spirit, for cleansing, Hebrews 9:13 but it rather signifies, that the blood of Christ, which cleanses from all sin, and answers to this purifying water, that its cleansing virtue is owing to Christ being made sin for his people; and that some may be instruments of directing souls to the blood of Christ for cleansing, and yet be defiled themselves: it does not appear that this man, thus unclean, was to have the water of purification sprinkled on him, but was only to wash his clothes; see Revelation 7:14

and he that toucheth the water of separation shall be unclean until even: but was not clean until he had washed, as Aben Ezra observes, though not expressed; for if one that only sprinkled it had need to be washed, much more one that touched it, and which was unavoidable, if, when he mixed the water and ashes together, he stirred them with his finger, Numbers 19:17, though Maimonides t understands this of sprinkling and touching the water when there was no necessity for it, when a person was not employed in doing the duty of this law.

t Hilchot Parah Adumah, c. 15. sect. 1.