The magic-users who rule Glantri don't like the idea of anyone challenging their monopoly on magic. As a result, it's against the law in Glantri to cast clerical spells unless you have a writ authorizing you to do so. Such a writ may only be provided by a noble in a position of authority — either a landowner or the head of a noble house.
Ordinary citizens wouldn't know a writ from a hole in the wall. Nobles, however, may be quick to demand to see a writ if they hear that you've been casting clerical spells. That's because if you don't have one, they can shake you down for cash in exchange for writing you one. (And if you say no, they can toss you into the oubliette until you agree.) Likewise, people who don't like you may try to blackmail you with threats of revealing your unlicensed spellcasting. In general, though, this law is rarely invoked; after all, few people want to get on the bad side of someone who has the ear of a god.
(This rule has been around from the beginning of the campaign, but it occurs to me that we've had enough time and turnover that most people either don't remember or never heard it to begin with. Hence this reminder!)