The Church of the Celestial Sun is the most common church in the human lands. It's origins are not well documented. Some time around six hundred years ago, when many humans were nomadic clans, the church started making a name for itself as wandering missionaries who offered their insight to clan leaders and the like. They called themselves Followers of the Sun. Soon all chieftains had one of these red robed monks in his court.
It wasn't until 325 that the first scriptures were recorded speaking of Zebulon and the other gods of the pantheon. It is believed among the Church historians that the original force behind the Followers of the Sun was the All Father himself. That he somehow gained deification upon his death.
His followrs formed the Church in response to the nascent kingdoms forming around them. The original sect, the Orthodoxy, established the infrastructure and religious administration. The next sects to be formed were the Brother Battle and the Circle of Amalthea as it became apparent in those early chaotic years that the Church could not rely on the kingdoms they served for protection and healing.
Perhaps as a knee jerk response to the arcane order the Avesti sect rose from the common lay parishioners that feared the arcane. Many of this order felt, at the time, that the Church was trying to cater to the nobility and had stopped caring for the commoners. This order took up the banner to protect and nourish the flock.
The Church has a robust organization. There is a Patriarch who governs with the council of Cardinals. They oversee the theological health of the Church. They are commonly known as the Red Priesthood. The Inquisition, teachers and political leaders within the Church come from this priesthood.
Parallel to the Red Priesthood is the White Priesthood. Members of this priesthood are most common and far more numerous. Lead by the White Cardinal, these priests see to the day to day operations and physical welfare of the flock in general. These priests are not celibate and may marry, unlike the Red Priesthood.
Many of the gospels speak of Zebulon preaching to humans around a sacred lake. For many years sages in the Church searched for this holy site. Through divine revelation the Church has claimed that the holy lake is in a mountain range now in the country of Kish. This remote site was claimed by the church in 600. They named the lake Tera and kept the name for the city that sprung up around it.
Since Kish has grown to surround the Church territories the Church has created teleportation circles in the fourth tier. These circles have sister circles in the historical capitals of the Human kindgoms. This network was created by the Eskatonic Order and is guarded by the Brother Battle The circles are used by Pilgrims who can afford it and nobility who can't be bothered by traveling through Kish.