If you go with corrupting emotions, providing temptations, visions, etc...
For ex, have the sexy denarian chick seduce the priest, and at some point, she wants to have sex in the church, but can't because it is still holy, wouldn't poochum please desecrate his own altar, for sexy ol' me?
And worse, she made a video of some of the highlights... So not only is there the desecration, but also the embarrassment of the parishioners. So instead of rallying around the church, they don't want to be associated with it.
If you want to get more sinister,
Pervert the good works of the church. The necromancers prey upon someone in the church food bank, conjuring up a hatred of the poor, or street people. They convince him it is his own idea to poison the food the soup kitchen hands out, to get rid of all the beggars.
Perhaps the church is known for its hospice care. The list of patients and their families is given to a bunch of vultures, selling funeral services, tacky memorial items, ambulance chasing lawyers and the like.
Might be interesting to have a scene afterwards, where some torn parishioner, who stuck with the church as long as he or she could, is informed that the troubles of the church started with actual malign intent, and wasn't simply the result of folly and ordinary human weakness.
Kind of daunting. If it was just ordinary human frailty at work, the parishioner could just move on to a different church. But few people expect to be tested like Job in the real world. To find that someone working with demons was doing cruel things just to get you to abandon faith in something. Especially if the parishioner then feels they failed, by NOT sticking with their church in its trials.
Got to give all those Champions of God some angst, you know?
