John Calvin Commentary Numbers 16:46

"And Moses said unto Aaron, Take they censer, and put fire therein from off the altar, and lay incense thereon, and carry it quickly unto the congregation, and make atonement for them: for there is wrath gone out from Jehovah; the plague is begun." — Numbers 16:46 (ASV)

And Moses said to Aaron. The expiation of so great a sin did not indeed depend on the incense-offering, nor are we to imagine that God is appeased by the fragrance of frankincense. A symbol was thus set before this hard-hearted people, by which they might be equally aroused to repentance and faith.

For however unreceptive they might be in their rebellion, yet the dignity of the priesthood was so evident in the censer, that they should have been awakened by it to reverence. Who would not view his ungodliness with horror, when he is made aware of having despised and violated that holiness in which the Divine power manifests itself for life or death? The sight of the censer might have rightly served to subdue their hardness of heart, so that at last they might begin to condemn and detest their wicked act.

The second warning which it gave them was no less profitable, i.e., that they might perceive that God was only appeased toward them through a mediator. But, insofar as the actual state of things allowed, the visible type directed them to the absent Savior.

Since, however, men corrupt and obscure the truth by their foolish inventions, His majesty is affirmed by the Divine institution of sacrifice, while Aaron, the typical priest, stands out, until the true, and only, and perpetual Mediator is revealed.

The verb כפר, caphar, properly signifies, as I have said elsewhere, to reconcile God to men through an expiation (piaculum; ) but, since here it refers to the people, the sense of Moses is rightly expressed by a single word, as one might say, to purge, or purify from pollution.