The common religion of the Isles is based on Sumerian cult and ritual. That is, the Lady equates with Inanna; her consort the Shepherd equates with Dumuzi; and the Sister fills the place of Ereshkigal, Queen of the Underworld.
Religion in the Isles (and generally, except perhaps in fantasy fiction) is separate from magic. The magic in Mistress of the Catacombs is based on the practice of the Mediterranean Basin in Classical times. The wellspring was mostly Egyptian, but there were admixtures from many other cultures (particularly the Jewish). What I’ve referred to as “words of power” are formally voces mysticae, words in the language of the demiurges who act as intercessors between humanity and the Gods.
I don’t myself believe in magic, Classical or otherwise, but I know that reality doesn’t always conform to my opinion of what it should be. Just to be on the safe side, I prefer not to pronounce the voces mysticae aloud.
As in the past, I’ve used Classical authors as part of the cultural underpinning of the Isles. Pendill is Ovid, who’s given me much pleasure over the years and has also educated me as a writer; Tincer is Tacitus, about whom I would say the same; and I was thinking of Gildas by the reference to Ascoin. I suppose a writer can learn from everything he reads, but I do hope that less of Gildas stuck than others.