“Then a man who is ceremonially clean must gather up the ashes of the red heifer and put them in a ceremonially clean place outside the camp. They must be kept for the community of the Israelites for use in the water of purification—it is a purification for sin.”
The Israelites were accustomed to think of water, with ashes of a sacrifice of a red heifer, associated with purification for sin. The sacrifice had a means of application to the impure person through the water.
Baptism brings these three together in the new covenant. In the water is applied the blood of Christ’s sacrifice, so that we might be cleansed from sin.