Numbers 8:7

Let them be sprinkled with the water of purification. This was the holy water, mixed with the ashes of the red cow, (Numbers xix.) appointed for purifying all that were unclean. It was a figure of the blood of Christ, applied to our souls by his holy sacraments. (Challoner)

Purification, (lustrationis) or "expiation." The water, mixed with ashes, was taken and sprinkled round about the houses, and upon those persons who wished either to be cleansed from some defilement, or to advance in virtue and purity. We use salt instead of ashes. Theocritus (Idyl. xxiv. 100,) puts these words in the mouth of Tiresias, "then mixt with salt, according to the law, with a green branch sprinkle the honoured and pure water, and sacrifice to the supreme Jupiter a hog, if you wish to gain the victory over your adversaries."

Flesh, to remind them that they must cut off all superfluous thoughts, the roots of which they will however never be able to destroy entirely, as St. Gregory (Mor. v. 3,) says, "The flesh always produces superfluities, which the spirit must always cut away with the sword of solicitude." See Leviticus xiv. 8., and xxi. 5, 10. (Haydock)

The priests serving in the temple were obliged to cut their hair every month; and the Levites probably observed the same regulation, to acknowledge, that they who approach to God, must be pure and detached from earthly cares, (Calmet) and particularly from the works of sin; to remind them of which, they were to be sprinkled with water, their garments washed, and they were to offer two oxen by the hands of Aaron, and to be lifted up or offered to God, to serve in his court. (Tirinus)