John Calvin Commentary Exodus 10:4

"Else, if thou refuse to let my people go, behold, to-morrow will I bring locusts into thy border:" — Exodus 10:4 (ASV)

Else, if you refuse. Moses denounces the extreme scarcity and famine of the land of Egypt, because the locusts will suddenly arise to entirely consume the remaining produce of the year, for half of it had already been destroyed by the hail.

But, although ancient histories bear witness, and it has happened even in our time, that locusts have devoured not only cornfields but also pastures, we may still gather from the circumstances that this was an extraordinary instance of divine vengeance. This is evident because Moses appoints the next day, relates that an incredible multitude suddenly burst forth, adds that such had never been seen, and finally, threatens that no house would be exempt from their invasion.

Moreover, it is worthwhile to remark again on the nature of the scourge: God collects and arms a host of vile insects, by which He may insultingly overcome this indomitable tyrant with all his forces.

The ingratitude of Egypt, too, was worthy of this recompense, since it was too great an indignity that the descendants of Joseph should be tyrannically persecuted in that country, which a little more than 250 years before he had preserved from famine by his efforts.

What follows in Exodus 10:6, that “he turned himself, and went out from Pharaoh,” is recorded as a sign of his indignation, as if Moses, worn out with the obstinacy of the tyrant, had hastily withdrawn from him without saying farewell. Therefore, although he was otherwise of a mild disposition, this peremptory harshness was to be adopted as a rebuke for the arrogance with which the tyrant spit in the face of heaven itself.

But let the Pharaohs of our age also learn that when they hinder the pure worship of God by their cruel threats, it is by His strict justice that fanatics, like locusts, assail their kingdoms with their impious errors and infect their people with contagion.