John Calvin Commentary Numbers 16:18

"And they took every man his censer, and put fire in them, and laid incense thereon, and stood at the door of the tent of meeting with Moses and Aaron." — Numbers 16:18 (ASV)

And they took every man his censer. It is evident how greatly they were blinded by pride, since, although admonished both by the confidence of Moses and by the previous examples, they still obstinately press on. Surely, if any spark of the fear of God had remained in them, their censers would have immediately fallen from their hands.

But Korah seems to have sought, as it were, deliberately how he might cast aside all fear and totally deprive himself of his senses. For in the next verse, Moses narrates how ostentatiously he hardened himself in his rebellion before he offered the incense. He gathered the people together to his side, so that the magnificence of his display might overwhelm the grace of God, which opposed him. In this, his senselessness is also clearly seen, when he seeks to fortify himself against God by the favor of the mob, as if he had desired to extinguish the light of the sun by interposing a little smoke.

Now, let us learn to condemn his folly so that nothing similar may happen within ourselves; for all ambitious persons are affected by the same disease. They gather their forces by trying to ingratiate themselves with men; and, if the world approves of them, they are intoxicated with such fatal confidence as to spit at the very clouds. But we will soon see how God, by a single breath, dissipates all their ungodly conspiracies.

On the other hand, the fickleness of the people is set before our eyes. For some time they had all been accustomed to the duly-appointed priesthood, which they knew to be instituted by God; yet only a single night is required for them to revolt to Korah. And, in fact, just as we are by nature slow to act rightly, so also we are carried away to evil in a moment, as soon as some villain lifts his finger.