I think actually the Harnic Pantheon could be re-interpreted to a theology with Monotheism at it's base with some pagan elements. A quick transmutation
of some of the religions -
The following are of a "christian" element, just various "sects" or preactices -
Save-K'nor - Monastics. Often in isolation, at least to themselves. They also represent the educated monks, who often were the ones who were able to record/write historical records. Much of the better detailed manorial agricutural output often comes from church run estates as a real world example. They would still be the "knowledgeable" ones.
Peoni - Various Friars, of whom some had a vow of poverty, often interacting directly with the lower classes.
Larani/Agrik - Get rid of the Balrogs, they could well be various sects within a religion, and often in opposition,
The others -
Halea - I like the idea of tying this to the Jewish religon, though one would either have to get rid of the Halean sexuality, or make this a more sexualized version of the Jewish religion. Origins in Karejia would make sense.
Ilvir - Druidism would make sense, though the presence of their god on Harn may not.
Sarajin - Typical Norse (pagan) religion.
Naveh - A bit tougher here. Perhaps a sect of Lythian Islam - but only a sect, and perhaps Harnians only exposure to Luthian "Islam", leading to a lot of wrong conclusions. The Navehans could be a branch the Middle ages assasin that were in the Middle East. Some Harnians might confuse this sect of "islam" with worship of Morgath.
Siem - Tough one for certain. Perhaps combine Siem and Ilvir into Druidism? The idea of "spirits" being all over (such as Asiri, Elmithri, etc.) would make this a very heretical religion. Of course the elves worship how they wish to. And this could even be a monotheistic belief in general - Same one god, though the "Siemists" give respect to all the woodland spirits, in many ways how the non-"pagan" Harnians worship their saints and prophets.
Morgath - Devil worshippers, or the evil deity who wrestles with the one god for control of men's souls. The only thing - I would think it would be difficult to have had worshippers of Morgath as a controlling faction as with Balsha, at least as open worshippers.
One would not need a "Christ" though, merely a monotheistic religion similar to Judaism or Islam, with various prophets and saints as part of the religion. The acknowledgement and worship and degree of worship given the various saints could well have a lot to do with some of the factions disdain towards each other.