I feel like this was written intentionally obscure so that DMs could make use of it for their specific campaigns as they liked.
That being said, I doubt wererats would have one world-religion and it would probably vary differently from region to region taking on various concepts, themes and trappings from the local surface-dwelling religions. Would be less like religion and more like various unconnected hedonistic or evil-aligned cults.
Yeah, it's also the first it was ever mentioned, as far as I know, unless they were in
Scholar of Decay, which I've never read. Richemulot never got much detail aside from that one novel.
EDIT: I had forgotten about the 2nd ed. game book
Monster Mythology, which had a god of wererats, Squerrik.
Squerrik (Lesser God)
The cowardly and physically weak Squerrik lairs in an endless series of burrows and tunnels, infested with traps, below one of the plains of Gehenna. He is a fearful creature who ever seeks protective magic, disguises, and items for keeping his enemies at bay (not that he has many; most other deities don't consider him worth bothering with).
Role-playing Notes: Squerrik feels little emotion for wererats and is uncaring about his own shamans, not sending any omens or warnings to them. Rarely, he may dispatch an avatar to lead a very large group of wererats out of their subterranean lairs to attack, loot, and pillage a surface community weakened by war, seige, or some other adversity. More often his avatars seek protective magical items for the god's use. The avatars are always cowardly and seek to escape combat if faced with a powerful enemy.
Statistics: AL le; WAL le (wererats); AoC thievery, disguise, concealment; SY rat's head with bared yellow teeth.
The Shamans
Squerrik's shamans may only memorize and use their spells in ratman form. They are not organized into any hierarchy, but lower-level shamans learn from higher-level ones and must show them respect and deference (and make offerings of treasures). Wererat shamans must be self-protective. They actively acquire protective magic and anything that aids their skills of concealment and disguise.