Augustine on Magic.
“In this model, magic appears in the context of the theory of signs as an act of communication with demonic powers (while Christian rituals are also acts of communication, but only with the divine sphere).
Thus, all superstitious practices, including divination and astrology, presuppose an implicit or explicit pact with demons. This is valid even in the case where the operator—deceived by the demons—is not aware of the pact, because this pact is secured by the magical language, signs, and rituals he has applied.
For a reader of Augustine, basically every instance of magic—however innocent it may seem—seems to be ultimately associated with idolatry and demonolatry, and becomes consequently harmful. Augustine was well aware of the common features and elements of the rituals of magic and those of religion (prayers, sacraments, and the cult of relics).
It is true—he wrote—that what magicians do is often similar to what saints do: the difference lies not in the visible realm but in what is secretly implied. While saints communicate with divine powers for the greater good, magicians seek their own, selfish ends.”
Benedek Láng, Unlocked Books: Manuscripts of Learned Magic in the Medieval Libraries of Central Europe, 2008: pg. 20.