John Calvin Commentary Numbers 20:23

"And Jehovah spake unto Moses and Aaron in mount Hor, by the border of the land of Edom, saying," — Numbers 20:23 (ASV)

And the Lord spoke to Moses. First of all, in the death of Aaron, we must consider the execution of the sentence by which he had been condemned. God wished to show that He had not threatened either him or Moses in vain with what then occurred, as children are usually threatened.

If Aaron had died without any such prediction, he might have seemed, due to his advanced age, merely to have paid what is called the debt of nature. In that case, the people might have been so overcome by their grief that they would have had no inclination to continue.

But now, with the condemnation of their public and common guilt clearly revealed in the death of one man, such great severity from God against the high priest—who had previously propitiated God for them all by his intercession—must have been a very sharp spur to them all.

For it must certainly have occurred to them that God was no longer to be trifled with, before whom not even this sacred dignity could escape punishment.

This was the reason Aaron was summoned to die in full view of everyone: so that the survivors might learn to live for God, as He instructed them to obey through this notable example.

For the rebuke—that they should not enter the land because they had rebelled against God’s word—was given less for the sake of Moses and Aaron, and more so that the people might perceive that they themselves deserved to perish ten times over. This was because their stubborn rebellion had provoked these holy men, so that in their excessive zeal, they had almost fallen away from the faith.