Centerra is powerfully monotheistic. Even the pagans of the sweltering north are not true pagans, and instead worship a heretical version of Hesaya called Celestialism. Even orcs worship Zulin, Prince of the Upper Air, in their own way.
(Zulin is the king of gods, and only kings are allowed to pray to him, but he is not the creator-god. Hesaya has no creation myth. The history of Centerra is too difficult to explain, even with resort to religion's trumps.)
The only people who aren't Hesayans are the Fire Cults, the ash-eaters, who burned their volcano gods rather than allowing them to be captured and assimilated into the Church. Their rejection of Hesaya is the reason that there are no volcanoes in Centerra (except for Lady Hellfire).
(In my mind, this is the biggest distinction between Warhammer's Church and Centerra's Church. In Centerra, even the monsters worship the same god; or at least, they quiver in fear when Zulin is mentioned, since they are keenly aware of their own status as unrepentant sinners.)
A secret tool of the Fire Cults, Double Fire is a special type of fire that only burns fire. (Of course, it generates enough heat to set nearby things on fire anyway, so the point is usually moot.) It cannot be made de novo, and must be kindled from a pre-existing flame of double fire. (This is similar to the holy fire of the Church, which is itself a reverse engineered form of double fire. There is a popular conspiracy theory that claims that Church avoids eradicating the fire cults in order to spy on them, in what can only be described as a religious form of industrial espionage directed towards the assimilation of new miracles and the expansion of Zulin's portfolio.)
By default, assume that a double fire is purest white. Compared to regular fire, it burns fuels twice as fast, and sheds light twice as far.
The older a double fire is, the hotter it burns and the faster it spreads. Older fires also blueshift along the blackbody spectrum.
The Fire Cults call it wrathful fire. In their hidden shrines, they incubate immensely powerful flames, hundreds of years old. These flame nurseries resemble bonsai gardens a little bit. Each flame is kept tiny in order to conserve fuel, but they are so powerful that some of them will devour a tree every day. Fire Cultists will also tell you that the older flames develop more idiosyncrasies, and even personalities.
These ancient flames are sometimes used offensively. The oldest of them burns ten times as bright, and ten times as fast, as normal fire. It doesn't take much imagination to see how they could be used to attack the Church.
At the very least, being burned alive by these old fires is blessedly quick: a grown man can be reduced to the finest ashes in less than a minute.
The Simurgh is Zulin's former wife. (She is not the goddess of birds. She is all birds, paradoxically.) She has a complex relationship with her ex-husband, encompassing the entire spectrum from blackest enemy to most treasured friend.
Zulin divorced the Simurgh in order to marry Ianu, his prophetess, after he lifted her bodily into heaven. This act also set the precedent for a moral divorce. (It had previously been punishable by death.)
They are given to those who make a promise to the church (see also: geas). They are made from heavy porphyry. As you stray from your vow, they grow heavier and heavier. If you die without fulfilling your promise, or if you lose the ring, it will pull your soul straight down to hell.
If the ring is removed, you can still feel it on your finger, invisible and heavy. The removed ring becomes non-magical.
(This is similar to the Silent Bell of St. Dormaine, in which the physical object is the "false" or "lesser" object, while the spiritual object is the real one, or at least, more real than the physical one. Although the Silent Bell lost its tongue decades ago, the faithful can still hear it calling them to prayer each morning. The bell's soul rings loudly, even if the bell does not.)
The Killing Wind
The four greatest servants of the Church of Hesaya are the Four Winds. The South Wind is known as the Killing Wind. It seeks and destroys the Church's enemies. It answers only to the pope.
It cannot be see, it cannot be fought. It deals non-lethal damage. If it would bring a creature down to 0 HP, it instead steals their breath, and makes that person intangible to air. (This feels similar to being launched into space, without any momentum. To the affected, the air becomes thinner and thinner until it is gone. You can no longer touch the air, because it has rejected you.)
The Killing Wind can be trapped in a room. (And for a while, it was. For 19 years, Gormazotz, king of the storm giants of Surlund, held the Killing Wind hostage in his dungeon. No southerly wind blew during this time.)
For this reason, the Killing Wind is loathe to enter any room where it might be trapped.
Depictions of Zulin
According to the doctrine of the Church, it is blasphemous to have any art that is not a depiction of Zulin.
Because of this, all of the illustrations in holy books are of Zulin. All people are Zulin (their identities indicated through the use of expressive hats). All trees are Zulin, and if you look closely, you can see that the leaves are small hands, each covered with the delicate blue skin of Zulin.
And among the crags of the background, you can see the geography of Zulin. The mountains are subtle noses, and the canyons are subtly ridged, as teeth.
As you'd expect, the only colors of ink that are allowed are blue and black. The exceptions are the unholy things, which are definitively not-Zulin, and so are illustrated in red. Demons, sinners, and satans.
Illustrations that do not conform to these very reasonable expectations are summarily burnt by the Fourth Lantern.
The 99 Satans
Although since the paladins conquered hell, Satan has been sundered into 99 parts. The 1st Satan is Asmodeus, the prime devil, who administers Hell on behalf of his paladin masters. The 63rd Satan is Orcus, who fights the paladin occupation alongside Jubilex (who is not a Satan).
(Satans replace balors in Centerra.)