Numbers 16:46

46.And Moses said unto 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 savor of frankincense; but thus was a symbol set before this grosshearted people, whereby they might be alike aroused to repentance and faith; for however insensible they might be in their rebellion, yet the dignity of the priesthood was so conspicuous in the censer, that they ought to have been awakened by it to reverence. For who would not view his impiety with horror, when he is made conscious of having despised and violated that sanctity wherein the Divine power displays itself for life or death? The sight of the censer might have justly availed to subdue their hardness of heart, so that at last they might begin to condemn and detest their unrighteous act. The second warning which it gave them was no less profitable, i.e., that they might perceive that God was only propitiated towards them by virtue of a mediator; but., in so far 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 fond inventions, His majesty is asserted by the Divine institution of sacrifice. Whilst Aaron, the typical priest, stands forth, until the true, and only, and perpetual Mediator shall be revealed.

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