John Calvin Commentary Numbers 19:6

"and the priest shall take cedar-wood, and hyssop, and scarlet, and cast it into the midst of the burning of the heifer." — Numbers 19:6 (ASV)

And the priest shall take cedar-wood. So that the sprinkling of the blood might be joined with that of the water, the cedar-wood, hyssop, and scarlet thread, with which the sprinkling was usually made, were cast into the fire. For unless the Israelites had been instructed by this visible sign, they would not have known so clearly that they were not only washed with the water, but that their uncleanness was also removed by the offering of the sacrifice.

But it was not enough that the blood should be poured out; unless, as has already been seen, they were purified by its sprinkling.

But, since the scent of cedar-wood is precious, and in hyssop there is a cleansing property, we gather from this also that the victim was pure, although it bore their sins together with the curse and expiation.

Peter teaches us how we are sprinkled with the blood of Christ, namely, through the Spirit (1 Peter 1:2). Indeed, John shows us in his Canonical Epistle that we find all the parts of this ceremony in Christ, where he writes that Christ came by water and blood, and it is the Spirit that bears witness, because the Spirit is truth. (1 John 5:6).