John Calvin Commentary Exodus 9:11

"And the magicians could not stand before Moses because of the boils; for the boils were upon the magicians, and upon all the Egyptians." — Exodus 9:11 (ASV)

And the magicians could not. Since the magicians were now also at hand, no doubt they were possessed by their former folly, so that they stood in readiness, as it were, in case an opportunity of contention should be offered them.

Indeed, since Satan, although ten times conquered, is still perpetually hurried forward with indefatigable obstinacy, so neither do his ministers desist from their madness, even though they have experienced how unsuccessful their battles are. These enchanters had recently confessed that their art could do no more, and yet they embolden themselves to try every extreme measure, until the disease of the boils drives them back in disgrace.

Therefore, so that we may not betray our madness by similar audacity, let us learn to give God His full glory by voluntary submission.

But that Pharaoh, when not only deprived of their assistance but even when abandoned and without their presence, is neither changed nor softened, proves that he was not so much deceived by the impostures of others as stupefied by his own malice and perversity; although Moses here repeats that “his heart was hardened by God;” because He desired, as if by an opposing barrier, to have an opportunity for manifesting His power.

And here the ignorance of those who imagine that God is endowed with mere prescience is refuted; for when “as the Lord has spoken” is added, He attributes both together to Himself, namely, the effect as well as the foreknowledge.

We will elaborate on this point a little further; yet let us remark that at the same time the tyrant was not absolved from crime, because his hardness of heart was voluntary. The blains, which were epidemic on the cattle, are a proof that not all of them died in the former catastrophe.