17 For the unclean they shall take some ashes of the burnt sin offering, and fresh[1] water shall be added in a vessel. 18 Then a clean person shall take hyssop and dip it in the water and sprinkle it on the tent and on all the furnishings and on the persons who were there and on whoever touched the bone, or the slain or the dead or the grave. 19 And the clean person shall sprinkle it on the unclean on the third day and on the seventh day. Thus on the seventh day he shall cleanse him, and he shall wash his clothes and bathe himself in water, and at evening he shall be clean. 20 "If the man who is unclean does not cleanse himself, that person shall be cut off from the midst of the assembly, since he has defiled the sanctuary of the Lord. Because the water for impurity has not been thrown on him, he is unclean.
Matthew Henry's Commentary on Numbers 19:17-20
Commentary on Numbers 19:11-22
(Read Numbers 19:11-22)
Why did the law make a corpse a defiling thing? Because death is the wages of sin, which entered into the world by it, and reigns by the power of it. The law could not conquer death, nor abolish it, as the gospel does, by bringing life and immortality to light, and so introducing a better hope. As the ashes of the heifer signified the merit of Christ, so the running water signified the power and grace of the blessed Spirit, who is compared to rivers of living water; and it is by his work that the righteousness of Christ is applied to us for our cleansing. Those who promise themselves benefit by the righteousness of Christ, while they submit not to the grace and influence of the Holy Spirit, do but deceive themselves; we cannot be purified by the ashes, otherwise than in the running water. What use could there be in these appointments, if they do not refer to the doctrines concerning the sacrifice of Christ? But comparing them with the New Testament, the knowledge to be got from them is evident. The true state of fallen man is shown in these institutions. Here we learn the defiling nature of sin, and are warned to avoid evil communications.