I posted a reply on this on Reddit but I'll post it here as well in case it was missed or someone else spots this and is curious. (And keep in mind this is all based on an understanding that I've gotten from the games, it's not 'official' or anything):
"Divinity" in Divinity is less about being classically 'divine' as we imagine it and more about being the most powerful person around.
Gods in Divinity aren't 'divinely mandated', they're not especially holy, they didn't spring from an expanse of nothingness to create existence. They're just extraordinarily powerful Sourcerers who are so much stronger than everyone else they might as well be gods.
TL;DR: There's no traditional 'holy' magic because there isn't. There's only Source, and those strong enough in Source are gods because there isn't anyone around who can challenge them enough to prove otherwise