J. Vilkka wrote:
From the beginning my intention has been to use Xianity as closely modeled after the medieval version as possible. That is, the claim that Christ was incarnated and lived as Jesus
somewehere in Kethira, is central to my Harnic Xianity.
And the "holy" book the Church uses, is Azeryani Vulgate, I guess. The equivalents of Greek and Hebrew will probably not be needed, but it would be nice to know if there are such equivalents.

Greek ~ Karejian, or possibly it's predecessor/progenitor Kalruia. Aramaic is tricky, but Besha might be taken as a rough equivalent? In the end it depends where you decide to locate the "Christ" figure's home.
J. Vilkka wrote:
Anyway, that's what I've been thinking. But I can still change much of this without problems.
I don't think theology will be a big issue in my campaign, only practical religion will, and the ability of the players (especially the mendicant's player) to make references to the religion, quotes from the bible perhaps, and such things. Religion is basically just scenery and furnishings, not the story or focus of the story.

Like Jim Chokey, I am wondering what impels you to choose this line of development. The potential pitfalls run deep. What, for example, do you plan to do about the Melderyni blockade and the Shek Pvar codes? Without the Concordat of the Illimitable Tome there is no justification for either, regardless whether the Concordat is 'real' or a feature of the collective Kethiran psyche stimulated by the existence of interworld 'gates' and the resultant perils.
I think you have to either remodel, or at least re-justify, several of the underpinning features of the Kethiran cultural fabric if the religious backdrop becomes 'Christian'. Do you plan to have other Terran faiths present, also (Islam, Judaeism, Paganism, Zoroastrianism)? What is 'special' about Christianity, and why would Christianity itself develop in the same way that it did on Terra in the absence of these influences on
it? If the core of your issue is that Christianity, not polytheism or pantheism or whatever, moulded the psyche that produced medieval feudalism then you have to account for the areas where the Kethiran milieu has apparently been affected by other influences on the Kethiran psyche, surely?
If what you want is really 'Medieval Europe with Fantasy elements' why not take a look at Ars Magica? They start with historical medieval europe and add fantastic elements - which strikes me as an easier way to approach such an endeavour. If, on the other hand, the you just have an issue with the fragmented nature of the Kethiran "pantheon", since it is neither monotheistic religions in competition nor a coherent, connected pantheon, why not develop/impose a structure of relations on the Kethiran gods or develop a (set of) monotheistic religion(s) with a decidedly Kethiran bent*?
I don't want to get overly pessimistic, here, but given the nature of your problem I think I don't see that your proposed fix is going to give you satisfactory results. If it does, on the other hand, then I'll be glad to have been wrong!
*: Maybe have the current 'gods' converted to 'prophets' - or even 'first tribal leaders' - with a Concordat imposed upon them by God? Maybe 'Old Testament' analogue = Save-K'nor/Siem/Halea (prophets), 'New Testament' analogue = Larani (prophet or saviour, depending on religion)/Peoni (Mary analogue), 'Newest Testament/Koran' analogue = Agrik (prophet - maybe who transformed into a fire spirit?). Morgath is a 'Satan' analogue, Naveh a heretical (to
everybody else!) prophet. Ilvir is the old heathen religion and Sarajin the foreign, semi-polytheistic religion of the Northmen. The Concordat stuff is in the 'Old Testament', together with other "God's laws" stuff about ethics and religious living. The slightly more nuanced philosophical ethics comes along in the 'New Testament' and a more 'individualistic' and 'realist' approach in the 'Newest Testament', maybe? The torturing, sadistic 'BBEG' Agrik I have long thought to be a heretical sect found on Hârn, in any case.