Church of Elwynn
The Church of Elwynn is the successor and continuator of the religious tradition of Treesian Unorthodoxy with a specific focus upon the eponymous goddess whose name has become indelibly linked with the land between the two rivers.
Ancient Elw religion was a dualistic religion between two neither fully good nor fully evil powers, Elu and Aqar. Elu was the god of the Elw (and those west of them), and Aqar the god of the others. Elu is related to Hebrew El (the Elw would therefore be a brother people to the ancient Ashkenazis), and Aqar is related to a Semitic word for "the other".
The Treesian Unorthodox Church came to Elwynn through missionary Hereton, who established the church here. When Shireroth became independent from Tymaria the church in Eliria was elevated to cathedral, with the whole of then Shireroth as diocese. Elwynn was after all the only place in Shireroth where TUC got a hold.
The Church spread well in Elwynn during the coming years. Churches were established all over the duchy, and the Church became associated with the ducal administration (as it helped with taxation, keeping of records, etc).
When Ardashir became baron, the church split from the TUC. So the CoE was born. Eliria's bishopric was elevated to an archdiocese, with dioceses being set up in the new cities. A few years later, the archdiocese split, following the ancient Elw principle of the rule of two, with the Diocese of Ardashirshahr elevated. Traditionalist adherents of the TUC stayed with the so-called Remnant Church.
Over time the Church took on more of the ancient dualistic tendencies and began to consider the gods of the Treesian pantheon, like those of the Cedrist religion to be distinct attributes of the dual nature of the divine. While keeping the Treesian liturgy, the actual theology became more dualistic, with Elwynn and Lest taking on the role and identity of Elu and Aqar respectively. Often, offshoots of the religion were eschatological, awaiting the union of Elwynn and Lest to bring about an end of the world as we know it.
The shift in theology brought the Free Elwynnese Church, which was at the beginning a pious but anti-liturgical movement in the Church (similar to pentacostalism in Christianity). However, as years passed by, the FEC became more mystical in its tendencies than the CoE. Of course, in Roqpin, the FEC camps are always a lot wilder than the CoE ones.
A major crisis for the Church came in the independence era. The Church was too associated (guilt by association) with "Free Elwynn" to be accepted in the East, and its churches and organisation was taken over by the Remnant Church until reunification when most congregations voluntarily returned to the CoE. However, the Remnant Church is still much stronger in the east than in the west (where it's almost unheard of).