This is accomplished mostly in this passage:
Many cultures around the world believed for millennia — and still believe — in witches. In typical folklore, past and present, witches are night-flying evildoers who inflict harm on others by supernatural means, such as curses, the evil eye, and magic substances. Witchcraft is usually thought of as an innate power, unlike sorcery, whose magical spells must be learned.
The example to keep in mind here is Circe, who bewitched and seduced Odysseus. She was supposed to be the daughter of the sun god Helios, so she would indeed have certain innate powers. In more modern terms, think of Samantha and her family from the TV show Bewitched. Somehow they were born with the inherent capacity for magic, only using incantations for the more dramatic spells.
It is only in the light of such examples that any sense can be made of the assertion
But to St. Augustine, concrete witchcraft consisted of idolatry and illusion rather than harm to others. Following Augustine, an anonymous ninth-century text, Canon Episcopi, became part of the Church’s canon law, declaring that belief in the reality of night-flying witches was heresy because there was no such thing as an actual witch.
In other words, to the extent that this is true, it is true only for a definition of witch that no one really uses today -- notably including modern adherents of Wicca, who are all humans, not literally demigods.
In fact, though, the assertion that the term "witch" means only a being like Circe or Samantha is not historically true; it has usually been taken to be synonymous with "sorcerer" or "sorceress". This is how it was used in Exodus 22:18:
Thou shalt not suffer a witch to live.Related passages include Leviticus 19:31:
Regard not them that have familiar spirits, neither seek after wizards, to be defiled by them: I am the Lord your God.and Deuteronomy 18:9-12:
When thou art come into the land which the Lord thy God giveth thee, thou shalt not learn to do after the abominations of those nations. There shall not be found among you any one that maketh his son or his daughter to pass through the fire, or that useth divination, or an observer of times, or an enchanter, or a witch. Or a charmer, or a consulter with familiar spirits, or a wizard, or a necromancer. For all that do these things are an abomination unto the Lord: and because of these abominations the Lord thy God doth drive them out from before thee.Witchcraft is clearly listed as an abomination that may be learned, not an inborn ability; it is, in other words, either identical to sorcery or the same basic thing with minor variations. Needless to say, St. Augustine would have been well aware of these passages, and St. Augustine never casually dismissed Scripture.
As for sorcery itself, here is what St. Augustine said about it in City of God, Book XXI, Chapter 6:
For to this inextinguishable lamp we add a host of marvels wrought by men, or by magic,— that is, by men under the influence of devils, or by the devils directly—for such marvels we cannot deny without impugning the truth of the sacred Scriptures we believe. That lamp, therefore, was either by some mechanical and human device fitted with asbestos, or it was arranged by magical art in order that the worshippers might be astonished, or some devil under the name of Venus so signally manifested himself that this prodigy both began and became permanent. Now devils are attracted to dwell in certain temples by means of the creatures (God's creatures, not theirs), who present to them what suits their various tastes. They are attracted not by food like animals, but, like spirits, by such symbols as suit their taste, various kinds of stones, woods, plants, animals, songs, rites. And that men may provide these attractions, the devils first of all cunningly seduce them, either by imbuing their hearts with a secret poison, or by revealing themselves under a friendly guise, and thus make a few of them their disciples, who become the instructors of the multitude. For unless they first instructed men, it were impossible to know what each of them desires, what they shrink from, by what name they should be invoked or constrained to be present. Hence the origin of magic and magicians.
Were there people wrongly convicted and executed for sorcery? Certainly. Also for treason, murder, and quite a few other crimes which used to be punished by death. But it is every bit as possible to be a witch as to be a traitor or an assassin. It is misleading to imply that St. Augustine thought otherwise.