The religion is monotheistic, centered around the worship of "The Watcher." The name of the religion as a whole is unknown; it may not even have one. Either way, it has split into two named branches, mirroring medieval Christianity and its schism into Catholicism and Protestantism.
The Jacoban branch believes the Watcher to be an angry, vengeful god and claims that Sims must live by strict moral code to avoid punishment. Getting Sims to submit to this code and fear punishment by the Watcher enough to want to be absolved is the main goal of Jacoban priests. Jacoban clothing and buildings are grand, especially their cathedrals.
The Peteran branch believe the Watcher to be a compassionate and loving god who wants Sims to have happiness. The faith is rather informal; priests often use humor in scheduled sermons and tend to engage in impromptu public sermons and evangelizing. Peterans are cerebral and seem to practice a form of mysticism, often praying for answers and writing down religious texts and tracts. Peteran buildings and clothes are visually more humble than Jacoban, with simple robes and monasteries.
It's important to note that, despite heavy inclination, Jacobans are not necessarily Evil and Peterans are not necessarily Good; a Sim's faith only indicates whether they think the Watcher is loving or vengeful. Thus, it's possible (if very unusual) to have a Good, Friendly, Insecure Jacoban priest and an Evil, Greedy, Misanthropic Peteran priest.
Both the Jacoban Priest and the Peteran Priest are playable Hero Sims. Female Sims can be part of the clergy of either branch. In addition, both branches' priests are allowed to marry and have children.
All Sims (except for priests) start out as Agnostics, neither believing in or disbelieving in the Watcher.
Priests can attempt to convert others to their faith, either by directly discussing it with individuals or giving sermons, which has a small chance of spontaneously converting Sims who listen to them. In addition, when being controlled, a hero can speak with a priest and ask to join their church, automatically converting the hero to the priest's religion.
Priests cannot attempt to convert rival priests away from their faith, nor do they get the option of joining the other priest's church.
Heroes who have a faith can approach the priest of their church to renounce their faith, turning them back into Agnostics. This doesn't harm their relationship with the priest, give either Sim a negative buff, or even remove buffs restricted to members of the faith they're renouncing.
Both churches receive regular donations from their followers in the form of getting a percentage of a religious Sim's income. Jacobans give 20% of their income while Peterans give 10%. It's unknown if these donations are mandatory or if Jacobans/Peterans voluntarily tithe their income out of fear/love.
Adult Sims can praise the Watcher while interacting with children and children can attend sermons and watch Peteran evangelizing. However, children cannot be affected by religion beyond this; children cannot be converted by priests, prayed with, or the target of any buffs from priest abilities.
Children will only belong to a religion if both of their parents belong to that religion.