Part of the Church's reputation is probably a holdover from Selene's outburst that got NeoSartania destroyed. Other factors that are Sorraine related:
1. Unwillingness to let other religions inside their borders - the only realm that does that currently.
2. Burning temples during the War of the Hearth Right gave the impression, even to their allies, that Sorraine was using the war as an excuse to further the Church's agenda.
3. Something that pissed Velax and Galiard off hugely was the perception that Sorraine was prepared to abandon Arcaea's and Cathay's forces in Haul when we needed them just so they could go burn some Zonasan temples.
4. Selene is doing herself and the Church no favours by threatening torture on every Ohnarian that ends up in her dungeons, as well as promising to continue fighting the war until both realms are destroyed, just as Velax is trying to negotiate a peace that sees Sorraine not losing a city.
