Inrithism

Inrithism is a (mostly) monotheistic religion that is currently considered by Imperial Religion as one of the biggest threats to its continued existence and power it has over Imperium of Karadia. It is mainly due to the expansiveness of Inrithian Church, which led to conversion of Nikopolan Hegemony and several smaller states in the region (for example Republic of Vycheav and human states on Sicilian Coast).
While it is considered as Vassal Religion of Imperial Religion, this is mostly one-sided. Inrithian Church considers the polytheistic faith of Imperium as a false one - and remains staunch in that belief. For them it isn't evil per se, and one that of course leads to an afterlife (and eternal one), but also one that limits what mortals can achieve and one that has to be eradicated via conversion. With only the true (or... truest) faith of Inri being the perfect one that leads to a perfect afterlife.
With Kingdom of Inrithia being a subject of Imperium (just as all other inrithian countries) the way of spreading their faith via more... forceful way was severed, ultimately forcing Church to accept several doctrines that cause some internal resistance to this day. Including the 'not evil per se and one that of course leads to an afterlife' part that is still rejected by significant part of Church. That has even more radical branch that actually condemns all supernatural as work of the Devils, though they are very rare.
Overwhelming majority of inrithians accept existence of daemons unrelated to their faith and the fact that other religions are, in a way, true. There are still some internal... incompatibilities regarding where exactly put them in the grander scheme of things.

Gods

Holy Trinity

The Holy Trinity of inrithism includes three separate gods of varying standings. This would generally made it a polytheism if not for the fact, that Trinity is considered a singular being. They are three... but they are one. Thus it is normally accepted as a monotheism.
The first of the Three is God The Father, also known under the name Yehavei he once used to introduce himself to ancient inhabitants of elven kingdoms of Aktaab and Senach. Which certainly isn't a real name, considering the fact that Yehavei in their ancient dialect means "Who I Am", literally taken from the phrase "I am who I am.". He is a central being of the inrithism, the Grand God (that by definition is exactly as much above normal polytheistic gods that they are above mortals). Considered (as all three beings of Trinity) a being perfectly good in a practically transcendent way.
He is also a creator of world (not only this but also every other than possibly exists) and all life within it. He made mortals in His image (with internal differences between species explained by the fact that He transcends the sheer idea of 'species' and is a being immaterial, the similarity being in the 'mental' way). He understands evil as well - while a being of omniscience that's a must - but He rejects it and expects the same from His believer. If ancient myths and teachings are to be trusted, He is a being of pure mercy... but also of justice, truly loving all of his Creations, but allowing them to reject Him if that's their decision.
Second part of the Trinity is Son of God. Also known under the name of Inri. His exact nature was a field of lots of controversions, that (for example) led to Hezranian Schism. According to Inrithian Church, He was a being made by Yehavei by merging a part of Himself with human nature. Born from mortal woman named Marha (believed by the Church to be a virgin, and a holy and sinless person) near the now holy city of Aktaab. His teachings regarding how one should live to gain access to Paradise became a foundation of moral teachings of Inrithism until today.
Due to the plot of elven chieftains (and human that invaded the area during His times) Inri was crucified, only to return from the Dead three days later, all according to what He prophesized earlier. This led to swift downfall of pre-inri religions in the area, which in turn lead to creation of modern inrithism.
Third part is a Holy Spirit. Which is... sort of a curio. While mentioned often in the teachings of Inri, he wasn't really properly explained, leaving a lot of space for interpretation. Today He is believed to be a general term for this small spark of divinity all mortals carry. The one that is responsible for the 'creative' aspect of their minds - mainly shaping the Dark and creation of all those supernatural entities and magic that roam the world as a whole, while also playing an important role in the inrithian story of world creation

Others

Despite the Trinity itself, there are also another beings that can be considered gods in the polytheistic (and magical) sense of the words.
First are the multitude of Saints, a mortals that lived a particularly pious lives (or were martyred for remaining steadfast in their faith in Inri). They can be considered an inrithian variant of Imperial Religion demigods. They are elevated to truly immense power via their connection to the Trinity and general belief in their holiness.
Unlike demigods, they aren't a target of the prayers, but are seen as a conduits that can deliver pleas of the mortalt to the God. Practically speaking they can influence Reality (being practically Inri's most trusted servants) and, in rare cases when Great Game actually requires it, actually descent and show themselves in Light physically.
They can be theoretically defeated by mortals in that form, but doing it is pretty much as hard as doing this to imperial demigods. Meaning that it'd require mobilizing small army of adventurers and magicians, or dedicated Chosen One with his equally dedicated entourage.
Second are the Archdevils. They are believed to be a wayward Archshepherds (servile daemons of Inri that decided that they detest mortals and decided to wage war against God. Of course, they lost (it was supposedly a rather curbstomp battle) and were imprisoned in Hell. Though, not fully. Variety of Devils, their servile daemons, actively oppose Shepherds, trying to corrupt mortals (whom they actually detest).
Archdevils themselves are bound in Hell by the Trinity, but - with mortals help - they can free themselves and roam the Light, which led to several devastating wars waged against the Faithful (as inrithians often call themselves).
In Hell they are believed to be equal to Imperial Gods, however they are banned from influencing Reality from there by God (a being infinitely more powerful that strongest among them), and with their power surpressed by Him even when they rampage through the Light.
Theoretically they are Avatars, while in fact they are limited to being comparable to a powerful demigods.

Theology

Creation Story

Creation story in Inrithism is a sort of a curio. The one that was granted to mortals by Yehavei in ancient times before Inri is mostly believed to be mostly metaphorical. The lack of predefined story of creation allowed inrithism (or, to be exact, Inrithian Church create their own, based on what they knew, the magical discoveries and more than 4000 years worth of philosophy and teachings of a various influential priests and monks.
God is ultimately believed to be a being truly primordial, that is older than both Reality and Unreality as a whole. He is simply older than Creation, meaning that He ultimately cannot be explained via its mechanisms... for He is the one who made them. Inri was made simply to help mortals improve and ascend from their pitiful existence, to search for something better. Higher. More... good. Exact reason for the Creation, however, is a mystery. To the point that some of the theologians reject its existence as a whole.
For them there is no Light, merely mortal souls (filled with the divine spark that Inri is supposed to call 'Holy Spirit') and the world they created for themselves via Dark that is a true state. There is no reality, merely a collective belief that it exists. Essentially one big school that is supposed to help mortals improve and ascend into higher state of existence (via purging their normally sinful natures with help of Inri that is required to achieve that) to the Paradise, to be able to sit beyond the Throne of God and understand the Creation as a whole.

History

Origins of inrithism is shrouded in mystery. It is believed that the old aktaab-senachian faith in Yehavei predates the collapse of Xylian Dominion. Most of the details, however, are far from being precise. Most of the records are scraps that survived the mess of Black Iron War. Aktaab and Senach were partially free even under late Dominion, which some attribute to the large area they occupied (and general lack of local xylian population) while others to the Yehavei and a series of disasters he brought on Xylian Dominion until they finally relented.
With most of Aktaab and Senach believing in Yehavei, they remained an elven curiosity (since they were really different than other elves) until human came. Then Inri was born. And new faith soon overtook whole region, completely extinguishing what remained of old faith. Ever since then, it spreads further, slowly overtaking new states (Nikopolan Hegemony and Republic of Vycheav being the most notable). With significant minorities that can be encountered pretty much everywhere in the Imperium of Karadia.

Afterlife

Inrithian afterlife is essentially divided into three parts. First is Paradise. It is a place of eternal happiness and joy, reserved for those who lived their lives as they should. Upholding the values that God expects them to, avoiding the sins He abhors and believing in Inri as their Messiah. It's details aren't specified, since it is believed that details just can't be fully described using mortal language.
Second is Purgatory, a place of tribulation and hardships, where souls of those that wasn't qualified to go straight to Heaven (but weren't evil as well) undergo proper purification, before they are allowed to enter the Heaven. According to general belief, those that managed to get into Purgatory can no longer be damned - they still have a hard and turbulent period before them, but are already saved.
Third is Hell. A place where Archdevils are imprisoned and were souls of sinners are banished as well. Most of it is pure nothingness where most of the souls reside in eternity, forever dreaming. Those that actually did something REALLY wrong spends eternity in a much worse way, tormented by devils and archdevils for their own amusement.