The Old Religion is a kind of blood magic that predates wizardry. It is mentioned in the second book by Jack Swift to Seph McCauley and then explained by Nicodemus Snowbeard as "an old kind of blood magic that predates wizardry."
History[]
The only known part of the Old Religion's history is that it predates wizardry, and thus the Weir and Faer which means that it is likely that this kind of magic can be practiced by Anaweir with proper training, or that there is a as of yet unknown magical guild.
Power[]
The Old Religion is a kind of blood/old magic, which makes it "more basic" than High Magic, requireing things like blood sacrafice.