that might one day become something worthy of a submission.
In almost every country (with perhaps the exception being a country with near-total lawlessness) the Religion of Thieves is going to be one that, however otherwise religiously tolerant the country is , has to stay on the quiet in the shadows. It has no written bible, but instead an oral one passed down through the priests/priestesses. They have no church buildings...at least no *open* church buildings. (Again, the sole exception might be in a lawless land, protected by it's acolytes.)
Old thieves, near the end of their lives, too old to steal and rob anymore, but with a wealth of experience who have dedicated their lives to the cause of crime. Unable to help physically anymore, they have turned to religion...their religion. They live on the offerings given to them by those who beleave. Inoffensive, they don't make waves, noone would have a reason to hurt/kill them. They have very little money, other then that needed for the basics of staying alive. If a thief murders or harms a priest/priestess, the God of Thieves will turn his back upon them, and it is said, his/her crimes will fail and land him/her in prison or worse. As well as criminal advice, they do the things of other priests in open religions....christenings, weddings, blessings, funerals and the like. The thief afterlife belief is that the God of Thieves will break a hole in the wall of Heaven for them, allowing them to enter despite their crimes.
Perhaps a broken key?
Every religious belief small or large has a code of conduct for it's devotees to follow. What sort of code would a Thieve's Religion have?