John Calvin Commentary Exodus 7:23

"And Pharaoh turned and went into his house, neither did he lay even this to heart." — Exodus 7:23 (ASV)

And Pharaoh turned. In this statement, Moses teaches us that the hardness of heart to which God had consigned Pharaoh was voluntary. Consequently, the sin rested with Pharaoh himself, and God's secret decree did not serve at all to lessen his culpability, for his folly is condemned because he did not set his heart to this also. From this it follows that he was the author of his own obstinacy because, being blinded by pride and contempt, he disregarded the glory of God.

Thus, the wicked, although as vessels of wrath they are cast by God into a reprobate mind, still harden themselves because they knowingly and willfully run against God. In this way, their complacency, audacity, and perverseness remove from them any excuse of ignorance or error.

Therefore, this example warns us not to be spiritually asleep when God arouses us, but to consider His works attentively, which can instruct us to reverence and fear Him.

The statement that the Egyptians dug wells for themselves increases the certainty of the miracle, as does the additional detail about the seven days. For if the corruption of the water had only been momentary, some suspicion of delusion might have arisen, which was removed by both its continued taste and appearance. Therefore, it was said before that the Egyptians would suffer inconvenience and pain⁹¹ from the lack of water; for I explain this to mean that they would be sorrowful and afflicted, that is, because they had nothing to drink.

⁹¹ He seems to allude to verse 18, which he translates “et molestia afficientur Aegyptii, bibendo aquas ex flumine.”