Charles Spurgeon Commentary Numbers 19:9

"And a man that is clean shall gather up the ashes of the heifer, and lay them up without the camp in a clean place; and it shall be kept for the congregation of the children of Israel for a water for impurity: it is a sin-offering." — Numbers 19:9 (ASV)

Shall gather up theashes of the heifer, and lay them up without the camp in a clean place, and it shall be kept for the congregation of the children of Israel for a water of separation: it is a purification for sin.

This ceremonial does not represent the putting away of sin (which was typified in the slaying of the victims); rather, it represents the daily cleansing which the children of God need, and the perpetual efficacy of the merit of Christ—for this red heifer was probably killed only once in the wilderness.

According to Jewish tradition, no more than six have ever been killed. I cannot tell whether that is true or not; but certainly, the ashes of one single beast would last for a long time if they were only to be mixed with water, and then the water sprinkled on the unclean.

So this ordinance is meant to represent the standing merit and the perpetual purifying of believers by the sacrifice of Christ enabling them to come to the worship of God, and to mingle with holy men, and even with holy angels, without defiling them.

In the fullest sense, it may be said of our Lord's atoning sacrifice, "It is a purification for sin."