I've alluded to the predominant religions of Arn and western Erida in a couple of places before, but I'll present them here in more detail. The two faiths are related historically and tend to be able to co-exist without much conflict, though this varies with time and place. As with any widespread religion in the real world, they are understood and practiced in a variety of different ways by adherents in different areas, but the essential elements are presented here.
The Church of Ascension
When Ahzuran achieved apotheosis and became God-Emperor of Old Thystara, he set in motion drastic changes in the traditional religion of the empire. The old gods were no longer seen as the unknowable creators of all, but instead as beings in a higher state. With a living God-Emperor, it was natural that his cult would become preeminent, and the cults of the other gods suffered as a result. It's possible that sectarian violence might have ripped the empire apart, and it certainly weakened it, but its dissolution was forestalled by the establishment of the Concordant. This allowed the continued existence of the old cults with some modification under the authority of the new church.
Authority within the church is nominally centralized, at first in the person of the God-Emperor, then later in the Hierophant, who is taken to be Noble Ahzuran's representative on Earth. The size and complexity of this task for those of less than Ascended capabilities often makes the authority essentially ceremonial, however. The major cults of the old gods (Seiptis, Æternus, Illumé, etc.) and the Ascended which arose in Ahzuran's wake (Ffalstagg, Illyra, etc.) have seats on the governing counsel who advise (and elect) the Hierophant.
The principle doctrine of the Church of Ascension is that man may achieve apotheosis by following the ancient paths rediscovered by Ahzuran. Acension is achieved by deeds which may be beyond the power of many, but piety will at least guarantee the faithful who don't ascend a place in the afterlife ruled by their patron Immortal.
Clerics of the Church of Ascension, not only pursue the paths of Ascension themselves (for the greater glory of the Church, of course) but aid other adventurers in this quest. They play a role in helping the church hierarchy determine the fitness of new godlings or entities encountered to be added to the Annals of the Ascended for the purposes of recognition and veneration.
The Issian Church
Over a century after Ahzuran moved beyond this plane, leaving his empire and church in the hands of mortals, a Thystaran man named Issus claimed to have a revelation. Issus proclaimed that, in a vision, Ahzuran and other great Immortals had shown him the truth--that Ascension was a state all men deserved. However, the arduous paths to Ascension, achievable only by a few, were not the true way this was meant to be done. Ascension only worked because the one true god, the solitary and increate Source of All, had made the multiverse in that way. Ascension wasn't godhood--just one a step closer to communion with the godhead. With faith and adherence to moral teaching, anyone could achieve that state--and more--upon death. The "gods" of the Church of Ascension, and the ancient cults, were re-conceived as saints, who were not to be worshipped, but venerated for the lessons they taught man through their life and travails, and the intercession in worldly events they might provide.
Issus is said to have been martyred (though the details of this is one of the church's mysteries) and to have ascended beyond any other. His teaching were popular and spread among the poor and disenfranchised of the Thystaran Empire. The nascent religion was unable to gain a significant foothold within the halls of power, and remains a small cult in its native land to this day. In the more rural colonies and provinces, the Issian faith proved more popular, particularly as the Empire began to decay. After the Empire's fall, Issianism became the preeminent religion of Western Erida--particularly in Llys and Staark.
The Issian Church is much less hierarchical than the Church of Ascension. Each Issian state had its own autocephalous hierarchy, but all recognized each other. This changed with the diabolic transformation of the Llysan branch of the church. The Issian Church of Llys transplanted to Arn is even less heirarchial with individual church's essentially asserting independence, though they tend to cooperate with each other.
Clerics in the Issian Church are interested in helping the poor and downtrodden as mandated by their belief (particularly those suffering under the yolk of evil (i.e. rebellious) ascended), and in expanding the temporal power of their church, both by proselytizing to the unfaithful, and filling the church coffers with treasure.
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