The Lore actually describes the Amurites inviting the Order in their land though, strongly implying that they make it their state religion. This was a purely pragmatic decision made by Valledia the Even, in order to combat the threat of the Infernal invasion. This was right after she freed a convicted necromancer and hired him to use some of the most evil magic in existence to torture the wife of Einion Logos far beyond the level that any mortal could normally survive. She did this in order to trick the Elohim into thinking that this crime had been performed by the Infernal, so that they would break their peace treaty and attack the Infernal before the Infernal could destroy the Amurites. Once divination revealed that the Infernal-Elohim peace treaty would not be broken soon enough, turning to the Order seemed like the best way to survive.
The Order and the Bannor in particular normally don't care much for sorcery, but they do use mages on occasion. The end of Lord of the Balors (if Capria wins) states that it was Bannor mages who severed the link between the Fane of the Lessers and Agares' Vault. It may be worth noting that the wife of the last king of the holy city of Old Braduk (when it was the capital of the Old Bannor Empire, which served Bhall when she was still good) was a Fire Archmage.