Auric Ulvin cannot adopt any religion.
He used to have the Agnostic trait (which was poorly named; in Casiel's case it means Deism, and in Auric's case it is Auto-theism--Ulvin is Mulcarn), but this was removed. Now, any leader that has a -100% preference for a religion is unable to adopt that religion.
I kinda wish he still had the trait though, since it makes it more obvious that he lacks the ability to convert and since it prevents him from wasting research on rushing to found a religion he can't adopt (He usually founds at least 3 religions in my games, one he founded 6 out of 7).
(actually the Agnostic trait doesn't stop Cassiel from founding the Empyrean or Council of Esus)