In my campaign as Egypt (VH/VH, no mods), I had a Jama and a Master Theologian’s Guild in Jerusalem. I had made a couple Imams and they started with 2 piety. Well I left them inside the city walls and over time they each got a monk and then some traits that increased their piety, even though they were not out converting heathens. I thought that the guild’s only improved agents at creation, but it seems that, at least with religious agents, they can improve them over time as well. I’m going to try this with merchants and spies/assassins, unless someone can confirm this.