Pharao hardened his own heart. By this we see that Pharao was himself the efficient cause of his heart being hardened, and not God. See the same repeated in ver. 32, Pharao hardened his heart at this time also; likewise chap. ix. 7, 35, and chap. xiii. 15. (Challoner)

This is the constant doctrine of the holy fathers, St. Augustine, ser. 88, de Temp. q. 18, 28, 36; St. Basil, orat., “that God is not the author of evil;” St. Chrysostom, hom. 67, in Jo.; &c. Hence Origen, periar. 3, says, “The Scripture sheweth manifestly that Pharao was hardened by his own will; for God said to him, thou wouldst not: if thou wilt not dismiss Israel.” Even the priests of the Philistines were so well convinced of this, that they said, (1 Kings vi. 6,) Why do you harden your hearts? God therefore hardened them only by not absolutely hindering their wickedness, and by punishing them with less severity, as they did not deserve to be corrected like dear children, Hebrews xii.

Perdition is from thyself, Osee xiii. 9. As cold naturally congeals water, so we of ourselves run to evil. Thus God cast Pharao into the sea, by permitting, not by forcing, him to enter, Exodus xv. 4. How shocking must then the blasphemous doctrine of Zuinglius, (ser. de provid. 5,) Calvin, (Instit. 8, 17,) &c., appear, who attribute every wicked deed to God, though they pretend at the same time that he is not unjust, even when he commands and impels a man to commit murder or adultery. Idem facinus puta adulterium...quantum Dei est auctoris, motoris, impulsoris opus est, crimen non est; quantum hominis est, crimen ac scelus est. (Zuinglius, sup.) The light of reason may suffice to confute such absurdity. (Worthington)