Numbers 19:2-9 ESV
“This is the statute of the law that the LORD has commanded: Tell the people of Israel to bring you a red heifer without defect, in which there is no blemish, and on which a yoke has never come.  And you shall give it to Eleazar the priest, and it shall be taken outside the camp and slaughtered before him.  And Eleazar the priest shall take some of its blood with his finger, and sprinkle some of its blood toward the front of the tent of meeting seven times.  And the heifer shall be burned in his sight. Its skin, its flesh, and its blood, with its dung, shall be burned.  And the priest shall take cedarwood and hyssop and scarlet yarn, and throw them into the fire burning the heifer.  Then the priest shall wash his clothes and bathe his body in water, and afterward he may come into the camp. But the priest shall be unclean until evening.  The one who burns the heifer shall wash his clothes in water and bathe his body in water and shall be unclean until evening.  And a man who is clean shall gather up the ashes of the heifer and deposit them outside the camp in a clean place. And they shall be kept for the water for impurity for the congregation of the people of Israel; it is a sin offering.
Eleazer the priest was supposed to make "the water for impurity", which was to be used to purify those who have been ceremonially defiled by touching the dead.
This is how the water was made: a red heifer without blemish was to be slaughtered outside the camp, its blood sprinkled at the tabernacle seven times, and the heifer completely burned to ashes. While it is burning, cedarwood, hyssop and scarlet yarn were to be thrown into the fire. And then these ashes were to be mixed with fresh water, and then used to purify a person who has touched the dead (Numbers 19:11-13), or even tents and the furnishings of those that have been defiled with the dead (v14-19).
It cannot be that water mixed with the ashes of a red heifer, cedarwood, hyssop and scarlet yarn had some 'magical' powers in itself to purify someone who has touched the dead. They are symbolic. The writer of Hebrews used the imagery of the red heifer being slaughtered and burned outside the camp to describe how Jesus Christ suffered outside the gate of Jerusalem, on Golgotha:
Hebrews 13:11-12 ESV
For the bodies of those animals whose blood is brought into the holy places by the high priest as a sacrifice for sin are burned outside the camp.  So Jesus also suffered outside the gate in order to sanctify the people through his own blood.
Jesus was the One "without defect", with "no blemish". Jesus was the One upon whom the fire of God's wrath sins came, for our sins. The blood of Jesus was the one that was offered, not towards an earthly Temple, but towards the heavenly one, so that it may plead salvation on our behalf. Cedarwood, hyssop and scarlet yarn probably symbolises the blood of Jesus on the wooden cross. And it is the Spirit of Christ -- often symbolised by water in the Scriptures -- who takes the death of Christ and actually applies it to our hearts, that we may be purified from an evil conscience:
Hebrews 10:22 ESV
let us draw near with a true heart in full assurance of faith, with our hearts sprinkled clean from an evil conscience and our bodies washed with pure water.
It is the knowledge that I have come in contact with that which is 'spiritually dead', that I have touched something spiritually unclean, something that originates from The unclean spirit, the devil (and so, even if I may not have committed the sin directly, the knowledge that I have become at least complicit of that sin just by being near it) is what we need to be cleansed from, every single day. What is the only thing that can wash us clean from this feeling of being unclean? The truth that Christ, the perfectly clean One, took the blame, and was slaughtered outside the camp for all our defilments. There is no other water but the Blood.
Let's pray this well-known prayer together:
"Almighty God, to you all hearts are open, all desires known, and from you no secrets are hidden. Cleanse the thoughts of our hearts by the inspiration of your Holy Spirit, that we may perfectly love you, and worthily magnify your holy name, through Christ our Lord. Amen."