[margin: Cassian, Conferences, collation 7, chapter 27; Gregory, Dialogues, book 1, chapter 4; life of St. Bernard, book 2, chapter 1]
I find two causes. The first is that in this way he believes he inflicts a greater injury on God, when he can abuse creatures endowed with reason, washed by baptism and anointed with chrism, to the contempt and provocation of the Creator himself. He also harms more people in this way, because human beings do not abhor conversation and familiarity with harmful-magicians as much as they abhor the company of the Devil. For if the demon always openly presented himself, many would refuse to strike a pact with him and practice these magical arts. Finally, he gains more companions in crime whom he may destroy.
The other cause is that he can do some things through human beings which either he could not do by himself or could not do so conveniently by his own immediate action. For example, if resistance is to be made to some holy man preaching or advising right things, he did this against Moses through Jannes and Jambres, and against St. Paul through Elymas. Likewise, if heresies are to be spread, he did this through Simon, Menander, Marcus, and others. Likewise, if sacred things are to be abused, which is very frequent in harmful magic, he does this through human beings, and with such things he gains a richer profit of souls. For in this way he draws more people into deception: he increases contempt in those who despise sacred things; he falsely persuades those who venerate them that these things are done by God, not by the demon, and thus he leads many to defend harmful-magicians and so fixes sin upon them. It also follows that, to magical superstition, human beings add sacrilege as well; therefore God, more and more offended, permits the princes of darkness more and more each day against miserable mortals.
Sometimes, however, what he could do through harmful-magicians he prefers to do by himself, both to win some favor from them, to remove suspicion of the harm from them, and to display his power in this way. Remigius illustrates each point with examples in book 2 of Demonolatry, chapter 9.
[margin: canon law, cause 26, question 5; Augustine, tractate 7 on John; Remigius, Demonolatry, book 2, chapter 9]
It remained to speak of the remedies for harmful magic; but I defer that to the final book. Next let us look at vain observance.