Gardner said Wicca was the remnant of a pre-Christian religion driven underground by the Church, and that the people tried as witches in early modern Europe had been its members. The idea came from the Egyptologist Margaret Murray, whose books of the 1920s and 1930s argued that the witch trials had persecuted a real surviving fertility cult. Murray selected her evidence heavily, and her thesis has been rejected by historians of the trials; Norman Cohn and Keith Thomas dismantled it, and no serious specialist now defends it.
The historian Ronald Hutton established where Wicca actually came from. His Triumph of the Moon (1999) traces its elements to Freemasonry, the ritual magic of the Hermetic Order of the Golden Dawn, Aleister Crowley, the folklore theories of James Frazer, romantic literature about a Goddess, and Gardner himself. The conclusion is not that Wicca is fraudulent but that it is new, and that its founders built it from materials available in their own century.
One figure needs correcting directly. The claim that nine million witches were killed is false. It originates with Gottfried Christian Voigt, who in 1784 took thirty burnings recorded at Quedlinburg over fourteen years and extrapolated that rate across the whole of Europe for eleven centuries; the American suffragist Matilda Joslyn Gage rounded his total to nine million in 1893, without evidence. Modern research based on surviving trial records puts executions across Europe between roughly 1450 and 1750 at something like forty to sixty thousand, with Brian Levack estimating about sixty thousand out of some hundred thousand trials, and men making up perhaps a fifth to a quarter of the dead.
Most Wiccan writers now say all of this themselves, and treat the religion as none the worse for being young.