John Calvin Commentary Exodus 8:18

"And the magicians did so with their enchantments to bring forth lice, but they could not: and there were lice upon man, and upon beast." — Exodus 8:18 (ASV)

And the magicians did so. The word “did” here means “they tried to do,” for they did not succeed, as soon becomes apparent. They are therefore said to have done what they attempted in vain, or what they tried to do, but without success. In this way, God took away from Pharaoh whatever excuse remained on the pretext of his being deceived. For although he had previously sought these deceptions himself, his obstinacy still had some plausible excuse as long as the magicians rivaled Moses in the contest. However, when he sees their art fail, he openly sets himself in opposition to God.

However, it was not with reference to Pharaoh alone that God restrained these impostors. He also exposed them to the ridicule of all, in order to assert entirely for Himself alone the glory of perfect power. From this, we gather how well, according to His inestimable wisdom, He restrains whatever freedom He temporarily allows the ministers of Satan. For when, by tolerating their boldness, He has sufficiently tested the faith of His people, He compels them to stop abruptly, so to speak, so that they sink in confusion and proceed no further, as Paul says when recounting this history (2 Timothy 3:9).