John Calvin Commentary Exodus 10:27

"But Jehovah hardened Pharaoh`s heart, and he would not let them go." — Exodus 10:27 (ASV)

But the Lord hardened Pharaoh’s heart. A probable conjecture may be drawn from this: at the coming of Moses, some light shone forth, so that the darkness was not so thick. For Pharaoh would never have dared to boast so proudly without being confident of impunity. His pretenses at the commencement (of their interview), however, are omitted here by Moses, though the mitigation of the horrible punishment that had urged him to supplication depended upon them.

But although he was still in a state of alarm, he nevertheless hardened himself and prepared for every extremity rather than simply obey God. Here, also, according to his custom, Moses asserts that God was the author of his obduracy. This was not because God inspired obstinacy in a heart otherwise disposed to docility and obedience, but because He gave over as a slave to Satan a reprobate who was willfully devoted to his own destruction, so that he might rush forward with still increasing pertinacity in his impiety.

But, since Moses has so often used this word, I am astonished at the boldness of certain sophists who, by substituting the word permission, allow themselves this frivolous evasion to escape so plain a statement.