Numbers 19:13 Whosoever touches the dead body of any man that is dead, and purifies not himself, defileth the tabernacle of the Lord; and that soul shall be cut off from Israel: because the water of separation was not sprinkled upon him, he shall be unclean; his uncleanness is yet upon him.
Touching a dead body defiles a person so that he may not come into the presence (tabernacle) of the Lord or live among the holy community. Of course, dead bodies had to be taken care of, so God made a provision for such people to be cleansed: they must be sprinkled with the water of separation. The water of separation, which is a purification from sin, comes from the ashes of a red heifer without blemish. The red heifer was burned with hyssop, cedar wood, and scarlet. This water was then used to purify, to make clean, the one who had been defiled.
To the NT believer, the significance of the ashes of a red heifer ritual is obvious: the Crucified Christ is the sacrifice that purifies from defilement and cleanses from sin. The emphasis on red points to the Gospel Truth: “the blood of Jesus his Son cleanses us from all sin.” Jesus is the sacrificed red heifer who by his shed blood makes the water of separation that separates us from the horrible defilement of sin and from its consequence, death. Death is so terrible, it must be fought against at all costs; Jesus wins the victory.
The whole picture of the ritual makes graphically clear for us what Jesus really did for us: He cleansed us from defilement as sinners deserving death, purified us with His blood to make us holy, gave life in place of death, and took away our sin to make us holy. The whole Mosaic Law makes us aware of how awful is our sin and how horrific is our death, so that we might the more appreciate the Gospel of Christ. Faith in Jesus deeply loves God.