Numbers 19:21 - Matthew Poole's English Annotations on the Holy Bible

Shall wash his clothes, because he is unclean, as it here follows. It is strange that the same water should cleanse one person, and defile another; but God would have it so, partly, to teach us that it did not cleanse by any virtue in itself, or in the work done, but only by virtue of God's appointment; partly, to mind the Jews of the imperfection of their priesthood, and their ritual purifications and expiations, and consequently of the necessity of a better priest and sacrifice and way of purifying, which these outward rites did point at; and partly, to show that the efficacy of God's ordinances doth not depend upon the person or quality of his ministers, because the same person who was polluted himself could and did cleanse others. He that toucheth the water of separation; either by sprinkling of it, or by being sprinkled with it; for even he that was cleansed by it, was not fully cleansed as soon as he was sprinkled, but only at the even of that day, as is said here and above, Numbers 19:19.