It's also Demeter, not Demitra.

The version of history presented in the song is a bit confused. The numbers are way exaggerated, but that was common folklore at the time and Murphy can't really be faulted for not looking deeper (the total death toll was still horrific). While the Pope may have started the witch-craze, he didn't subsequently control it, it wasn't just done by the Inquisition, and the Protestants were just as bad at persecuting witches in the areas they dominated (like Scotland). The main text for witch-hunters was the Malleus Maleficarum, which the Inquisition rejected on theological grounds.

It is also rather dubious whether most witches believed anything like the Wiccan gospel behind the song. Carlo Ginzburg's Ecstasies gives a much more complicated picture, and there are large parts of Europe where "witches" must have believed things that even he didn't identify.