So you have a highly traditional and conservative religion, one that doesn't embrace change or advances in magic or technology. It all depends on the light you want to show them in. If you want them to be sympathetic, let them keep their dogmatic ways but be honestly good people, even if during the height of their power their faith was more tyrannical. Also, they could be similar to modern groups such as the Amish, forsaking advances and such as this weakens their community, and promotes violence and warfare, and hard labor is homage to God.
The alternative could be a seething violent side that sees change as evil and that it should be repressed. Since their defeat, they have a primitive lifestyle, enforcing their faith on one another, ruthlessly stamping out alternate creeds and splinter sects, and waging a guerilla war against the new standing faith. This could be a simple grassroots movement appealing to the masses, or it could literally be war, burning churches, capturing and killing clergy of the dominant faith.