John Calvin Commentary Exodus 10:22

"And Moses stretched forth his hand toward heaven; and there was a thick darkness in all the land of Egypt three days;" — Exodus 10:22 (ASV)

And Moses stretched forth his hand. By this darkness God not only wished to reprove the blindness of Pharaoh’s mind, but in every way to convince him how senseless and irrational he was in his resistance.

There is no blessing more common to all people, from the very highest to the lowest, than light, which is enjoyed no less by the humblest and most lowly people than by the greatest kings. It was, then, a terrible judgment of God that the whole world should be enlightened by the sun’s rays, while the Egyptians, although possessing sight, were plunged in darkness.

What madness, then, could be greater than theirs, when in their hardness of heart they do not cease to contend against God’s hand, as formidable as it was? Their waters turned into blood had denied them drink; frogs and other animals had filled the whole country; they had almost been consumed by lice; their limbs had been fevered by boils; the hail had destroyed part of their grain; the locusts had brought even greater destruction. Even rocks and stones should have been somewhat terrified by such warnings.

This admonition, then, was very timely, namely, that darkness should be spread over all Egypt, so that they might understand that, when God was angry with them, the very hosts of heaven were armed against them. And, so that God’s vengeance should be neither obscure nor doubtful, the cause of the darkness could not be attributed to an eclipse, both because of its density and the time it lasted. For both of these circumstances are expressly noted by Moses, so that it might be clearer that the sun was obscured to the Egyptians because they had attempted to extinguish God’s glory by their impious contempt.

On the contrary, the Israelites must have gained new cheerfulness when they recognized in the sun’s brightness that God’s paternal countenance was shining upon them; for He then enlightened them with His favor, as if to show them the freedom of their departure. And, indeed, He might have at once led them out from their astonished enemies; but He chose, as we will see, to prepare their departure in another way.