John Wesley on Numbers 19:9

For a water - Or, to the water, that is, to be put to the water, or mixed with it. Of separation - Appointed for the cleansing of them that are in a state of separation, who for their uncleanness are separated from the congregation. It is a purification for sin - Heb. a sin, that is, an offering for sin, or rather a mean for expiation or cleansing of sin. And this was a type of that purification for sin, which our Lord Jesus made by his death.