John Calvin Commentary Exodus 9:19

"Now therefore send, hasten in thy cattle and all that thou hast in the field; [for] every man and beast that shall be found in the field, and shall not be brought home, the hail shall come down upon them, and they shall die." — Exodus 9:19 (ASV)

Sealed therefore now. He does not give this counsel as if He would spare His professed enemy, but He insults his mad confidence, because until now in his complacent security he had despised whatever punishments had been declared against him. He indirectly hints, therefore, that now is the time for fear.

Secondly, when God contends, the outcome is not uncertain; because He not only openly challenges him to combat but assures him that He will have no difficulty in routing him.

Finally, He shows him that He has no need of deceit or any stratagems to overtake His enemy, but that, although He grants him a way of escape, He would still be victorious.