John Calvin Commentary Numbers 20:12

"And Jehovah said unto Moses and Aaron, Because ye believed not in me, to sanctify me in the eyes of the children of Israel, therefore ye shall not bring this assembly into the land which I have given them." — Numbers 20:12 (ASV)

And the Lord spoke to Moses and Aaron. God here both sets forth their crime and pronounces its punishment. Now, while unbelief is in itself a gross and detestable evil, God aggravates its guilt by declaring its consequence, namely, that He was deprived of His glory when Moses and Aaron, who ought to have been the proclaimers of the miracle, were, as it were, confounded with shame. For while their confidence, by exciting attention, would have sanctified God’s name, by their mistrust it happened that all were led to think that there was nothing to be hoped for from His assistance.

When Moses not only sincerely confesses his guilt but also relates how he was condemned by God, and, so that his disgrace may be more complete, introduces Him speaking as from His judgment-seat, this greatly helps to establish the truth of his doctrine.

For what human being, unless he had renounced all carnal affections, would voluntarily endure to declare himself guilty before all the world? His angelic virtues were sufficient to exempt him from all suspicion. Having erred in one particular only, he proclaims the disgrace which he might have concealed and does not hesitate to disparage himself to magnify the goodness of God.

And surely it is obvious from the passage that, whenever God had previously pardoned the people at the request of Moses, the pardon was no less gratuitous than if he had not interceded for them. For the intercession of Moses ceases on this occasion, yet God does not fail to deal kindly with them in their unworthiness, as was His custom.