John Calvin on Exodus 9:19

19. Sealed therefore now. He does not give this counsel as if he would spare His professed enemy, but he insults his mad confidence, because hitherto in his supine security he had despised whatever punishments had been denounced against him. He indirectly hints, therefore, that now is the time for fear. Secondly, that when God contends, the event is not a doubtful one; because He not only openly challenges him to the combat, but assures him that He shall have no difficulty in putting him to the rout. Finally, he shows him, that He has no need of deceit, or of any stratagems to overtake His enemy, but that, although he grants him a way of escape, still He should be victorious.