John Calvin Commentary Numbers 19:11

"He that toucheth the dead body of any man shall be unclean seven days:" — Numbers 19:11 (ASV)

He that toucheth the dead body. He now describes certain forms of pollution for which washing was necessary. All of them, however, come to this point: that people are defiled by the touch of a corpse, bones, or a grave.

Nor is there any distinction made here between the body of a person who is slain and one who has died in bed. From this, it follows that death is presented here as a reflection of God’s curse.

And certainly, if we consider its origin and cause, the corruption of nature, by which the image of God is marred, presents itself in every dead person. For, unless we were altogether corrupt, we would not be born to perish.

But God also taught His people in another way that uncleanness is contracted through our association with the unfruitful works of darkness.

For the Apostle (Hebrews 6:1) calls them dead works, either because of their consequences or because, just as faith is the life of the soul, so unbelief keeps it in death.

Since, then, the corpse, the bones, and the grave designate whatever we bring from the womb—because, until we are born again and God gives us life by His Spirit and faith, we are dead while we live—it is clear that the children of Israel were reminded to keep themselves pure before God by abstaining from all corruption. For if they were rendered unclean by their contact with a dead person, they had to immediately resort to ceremonial washing.

In summary, the ceremony had no other object than that they should serve God in purity from the sins of the flesh and engage in constant thoughts of repentance. And if they fell from their purity, they were to strive to obtain reconciliation with God through sacrifice and ceremonial washing.