The Church of Vaarta
There is no actual name for the religion of Vaarta. If you are a Vaartan you are automatically a follower, if you convert you automatically become a Vaartan, it is simply the one, true religion, the Church. Its main headquarters, where the Patriarch is based, is at Medinavic, a large heavily fortified religious settlement, near Novomsk.
Vaartans believe that God had two sons, Kebel and Markam. Markam is destined to be the saviour of the world, to smite the unrighteous and to lift up his devoted followers to heaven and eternal life. Kebel is the Tester, the Wicked One, the Tempter - sent to expose the evil inherent in mankind, to lead the unrighteous to their doom, to test the righteous in their faith, he must be constantly fought.
When God created paradise he caused His Children to be born. The two looked upon the savage humans and fought for dominance over them. Markam the eldest, believed that they could be educated and become great and benevolent thinkers whereas Kebel saw them as nothing more than toys for his own amusement, base creatures with no redeeming qualities. Markam was badly wounded in the fight - a cut to his head, his left hand severed, a deep wound in his belly and one on his thigh - but he managed to escape. While Kebel slept, Markam gathered the people to him in order to lead them out of paradise and into the world and protect them from his brother.
When Markam died his blood was caught in a chalice and drunk by Iantash, the first Patriarch. Markam's soul returned to God's side to await the time for Him to be reborn and lead the people to salvation. Meanwhile Kebel is extremely angry that the people escaped to the world where His power is less.
This religion is a harsh, monolithic, reactionary and slightly bloodthirsty organisation. It is technically forbidden for priests to play a part in politics, which is probably why they have never tried to rule directly or get rid of the Khazar. Unofficially they are as power hungry as everyone else and dabble shamelessly in politics. Heresy and heterodoxy are ruthlessly crushed whenever they appear, for example the Yavennik Heresy.
The Book of Iantash is their holy book, which also contains later sections by other saints and prophets. The Patriarch is the leader of the Church and he drinks the blood of his predecessor so that the Blood of Markam is directly transferred. He is called the Holy Father, all other priests are addressed as "Yod", meaning Brother.
The actual history of the Church begins in the wide plains of the outer sections of what is now Vaarta, it was a local religion that gradually expanded, helped considerably when Tovar, a Kizen of Novomsk converted and went on to conquer the rest of the Vaartan lands. Other human cultures are seen as the descendants of outlaws and heretics that Kebel has lead astray. Non-human sentients are seen as at best sub-human (like the Salsham'ai) created to be slaves for Vaartans and at worst the seed of Kebel (like the Tlaxu) who should be wiped from the face of the world. Serfdom is justified because they believe that all nobles are the descendants of Markam, who mated with many human women before his battle with Kebel (This incident has meant that infidelity has tacit approval from the church, for noble men that is. Women found to be adulterous often claim in their defence that their lovers claimed to be Markam in disguise - and they could not refuse the Son of God). Nobles are suppose to care for and protect the serfs under their control but in practise many abuse their privileges to different degrees.
Monasteries are the only places of advanced education in Vaarta so most young men of the upper classes are sent there in their youth for a few years. Anyone wanting to become a full time scholar would have to join the priesthood. The priesthood are male celibates clad in black with those tall round hats. Iconography is a highly developed art form.
Sorcery is strictly forbidden on pain of death, although the priesthood has some small mastery of exorcisms and anti-demonology.