If they're SA and stopping the divine reconquest of the faith's most holy city they should be excommunicated.
The risk is entirely to the priest aside from some loss of followers.
You're forgetting that Astrum also has ESA members who would not allow it, plus this wasn't a religious war, that was made very clear by the SA council. To make things worse, Astrum was actually now allowed to even travel through Caiyun for the duration of this war by the SA.
