The magicals of the Western Isles did not believe in the old gods.
They believed in Magic in much the same way non-magicals believed in electricity – it was a power that could be used for good or ill. Like electricity, there were 'rules' to use it safely and failure to follow those rules could result in catastrophe. And also like electricity, few considered Magic to be more than simply power. Power to be wielded to change the world more to their liking with the limits of the power to be used dependent only on the personal strength of the wielder.
They considered the ways of their ancestors to be archaic and primitive. They eschewed the rituals of the circle of the year, cutting themselves off from the cycles of renewal and the power of Gaia and the Elements. They denied the magic native to women and the magic of those not bound by simple definitions of gender and forced their children to use the magic of men instead of the magic of their birthright.
They denigrated the paths that other beings took – the fae, the mer, the goblins, the centaurs, the elves. Even family magic was demonized. The ability to speak with serpents or plants or water creatures or even the land itself was deemed 'dark' and 'evil'.
The magicals of the Western Isles did not believe in the old gods. They believed that they were gods and they refused to believe that Magic herself was willing and able to to give her blessing to anyone outside of their own numbers.
Needless to say, Mother Magic was pissed.