John Calvin Commentary Numbers 21:6

"And Jehovah sent fiery serpents among the people, and they bit the people; and much people of Israel died." — Numbers 21:6 (ASV)

And the Lord sent fiery serpents. Their ingratitude was justly and profitably chastised by this punishment; for they were practically taught that it was only through God’s paternal care that they had previously been free from innumerable evils, and that He possessed numerous forms of punishment by which to take vengeance on the wicked.

Although deserts are full of many poisonous animals, it is still probable that these serpents suddenly arose and were created for this special purpose; as if God, in His determination to correct the people’s pride, were to call new enemies into being to trouble them. For they were made to feel how great their folly was to rebel against God when they were not able to cope with the serpents. This, then, was an admirable plan for humbling them: to contemptuously bring these serpents into the field against them and thus convince them of their weakness.

Consequently, they both confessed their guilt and acknowledged that there was no other remedy for them except to obtain pardon from God. These two things, as we are aware, are necessary to appease God: first, that the sinner should be dissatisfied with himself and self-condemned; and, secondly, that he should seek to be reconciled to God. The people seem to faithfully fulfill both these conditions when they, of their own accord, acknowledge their guilt and humbly turn to God’s mercy.

It is through the influence of terror that they implore the prayers of Moses, since they consider themselves unworthy of favor unless an advocate (patronus) intercedes for them. Indeed, it would be erroneous for those who are conscience-stricken to invite an intercessor to stand between them and God unless they also unite their own prayers with his; for nothing is more contrary to faith than such a state of alarm that prevents us from calling upon God.

Still, the kindness of Moses and his accustomed gentleness are perceived in this: that he is so readily disposed to listen to these wicked people. God also, on His part, shows that the prayer of a righteous man is not unavailing when He heals the wound He had inflicted.¹²¹

¹²¹ Addition in Fr., “si tost;” so speedily. “si tost;” so speedily.