The Builders are modeled after an exaggerated kind of Catholicism, specifically around the time of the Inquisition. The Builder Church is deeply intertwined with the politics of the Empire, and the bishops of the Church vie for power with the most powerful nobles. The Church does not officially control the city, but it does have the authority to create and enforce laws relating to orthodoxy and heresy, as well as try its own members in ecclesiastical courts.
The Builder Religion
The Builders believe that it is man's ability to build and create that sets him apart from the beasts. Man was given this gift, along with fire and tools, with the commandment to "take dominion" over the earth. To the Builders, nature is chaotic and pointless. Human civilization, with roads, walls, and aqueducts, are all elements of order, which honour God. It is man's duty to build, to create order, and to spread civilization. Anyone that opposes the spread of civilization (as Pagans often do) is working against that purpose.
The Builders believe the afterlife is a gleaming, golden city. The city is perfect in every way, running itself like clockwork. The more you worked for God in this life, the better your lodgings and the greater your freedom. Particularly holy men (and rarely, women) are considered Saints after their death and are given the best positions within the Holy City. Because Saints have important positions, people often pray to them directly, especially on behalf of loved ones who have died.
Those on earth who did not sufficiently honour God with their works on earth wind up in purgatory--a dreary place where they can work their way into heaven through hard labour. Those who die unrepentent or with too great a sin are barred from The Blessed City and are dragged off by demons (who are often depicted as looking like pagan nature-gods) to the underworld--a chaotic realm where they are tormented for all eternity.
Builders value order, hard work, and devotion. Destruction of property, sloth, and chaos are all great evils. Builders believe in demons and evil spirits that try to corrupt mankind. These evil spirits are often believed to live in or be connected to nature, and can be summoned by evil thoughts. The spirits whisper into the ears of those who will listen, tempting them to be slothful or do other sinful things (like destroy property). Theft of property is also high on their list of sins.
Builders don't have confessions, they instead have "ordeals" people must go through to rid themselves of sin. These ordeals can range from helping to build a cathedral to having to carry hot iron across coals. The rich can, of course, offer donations to the Church in lieu of the ordeal. Sometimes Builders will force these ordeals on people, especially heretics, for the good of their souls. Other people mistakenly see this as 'torture'.
Hard work (slave labour) is also seen by the Builders as literally good for the soul. Sinful people can earn a heavenly reward by working hard, even if they aren't exactly doing the work willingly. The Builders often put prisoners and Pagans to work in what are essentially slave labour camps, in a sincere attempt to save their souls.
Amos is the main prophet of the Builder religion, and is seen as a Moses-like figure. He was given the "One True Hammer" directly by the Lord Builder, according to scripture, and laid the foundation for the first Builder Church in the Holy City thousands of years ago. The Holy Book of the Builders is called the "Book of Amos".
The Name of God
Since the Builders believe in a single god, they often refer to Him simply as, "God" (as in, "God bless thy works"). Just like in our own history, however, there are many other names used for God, including, "The Almighty", "The Lord Builder" or, "The Master Builder". The more formal the occasion, the longer and more elaborate the name tends to get (eg. "The Almighty Lord Builder, Master and Creator of All Things").
The head of the Builder Church is the Patriarch, who lives in Sancta Civitas, the Holy City. The Patriarch has far more political influence than anyone else in the western world--even more than the Emperor himself.
The Builder Church spans the old Empire and even beyond. The Church divides its territory into dioceses (also called bishoprics) and each of these is ruled by a bishop or archbishop. These powerful figures are the equivalent of a baron or duke.
Below the bishops are the Builder Priests, who usually manage a single church or cathedral. Priests can have a lot of influence on a local scale, and are just as important in political circles as wealthy merchant families and minor nobles.
Below the priests are the cathedral guards. They defend Church territory, assist in establishing new monasteries, and sometimes enforce Church law. This position is a popular one with the lesser sons of nobility.
Beneath the builder guards are acolytes, those who have taken their oaths but are not full members of the clergy. They are given servile tasks, such as transcribing books, assisting in the forge, and labouring on building projects.
The Inquisitors are a separate order of Builders with their own internal structure. They are the keepers of orthodoxy, and handle the interviews and trials of all those charged with heresy. Most judges of ecclesiastical courts are also Inquisitors. The Inquisitors answer only to the Patriarch (the head of the Church) himself.
In addition to the Inquisitors, there are also monastic and military orders in the Builder Church. Monastic orders tend to live contemplative lives in isolated monasteries, reading and writing books and studying ways to glorify God with even greater works. Military orders (such as the Knights of the Hammer) organize themselves the same way, but focus more on fighting barbarian tribes, fortifying Builder outposts and guarding travelers upon Builder Roads.
Builder Churches are usually grand affairs, full of gothic spires, flying buttresses, and decorative statues. Metal is a symbol of man's dominion over the elements, and metal (especially wrought iron) is heavily used in Builder architecture. All churches of reasonable size have a forge attached to them, where ceremonial tools are made and iron is heated for ordeals.
The symbol of the Builder Church is a hammer and gear blended together, and these two motifs are frequently used in their decorations. The hammer is a symbol of power and strength, and the gear a symbol of perfection and eternity.
The Church is always continuing its mission to tame and civilize the land. Just as The City continues to expand, new monasteries are formed in the wilderness, to protect and cultivate the nearby resources (Builder monasteries are more like small fortresses). They are the only places outside the city where a civilized man can feel safe, and they are therefore heavily used by merchants, mercenaries, and any others who cannot travel by ship. By collecting tithes from these travelers, the Builders gain a great deal of wealth to finance future building projects.
When Builders speak formally they use an archaic form of English, with "thee", "thou" and other biblical-sounding conventions (eg. "Wherefore smitest thou thy fellow?"). The more formal the occasion, the more strongly this archaic language is used. Some builders adopt this speech even in more casual day-to-day affairs, especially if they are older or were raised by the Church. (See here for tips on how to properly use archaic English conventions like thee and thou)
Views on Other Factions
The Church sees most magic as sinful, since it involves (in their minds) dealing with evil spirits. Only certain kinds of magic are legal to perform in the city, including healing and protective spells. Divining the future, destructive spells or those that bend the will are all considered heretical. Of course, magic-like abilities, like priests throwing holy fire, come from God and are not considered magic.
The Church see Pagans as--at best--misguided, primitive souls who have been corrupted by their close contact with spirits and nature, or at worst, active agents of chaos bent on destroying civilization. Simply being a pagan is not a crime, but pagans are frequently blamed for anything negative that happens (plagues, fires, etc) and are seen as easy targets for the Inquisition.
The Church has a strong relationship with craft guilds, especially those that deal with metal, stone or machinery.
Builder monks occasionally dabble in invention and mathematics, but they are not engineers. If they want complicated machinery built, they must hire the appropriate guild like anyone else. Builders embrace clockwork devices, steamworks and (to a lesser extent) electricity. Historically, many Builder monks have been involved in discoveries related to these kinds of technologies, and the church enthusiastically spreads the knowledge of their creation as gifts from the Master Builder Himself.
The Builders do not rule over cities or secular territory. They have authority in religious matters, but in Bridgeport the Lord Mayor and civil courts handle secular laws (there is often debate over which is which, however).
Other than Pagans, virtually everyone is a follower of "The Faith". This is a deeply religious, pre-enlightenment kind of society. There is a difference between being a faithful churchgoer who believes in God and goes to Mass, and "being a Builder", which means you belong the clergy in some way.