The devil wants to be a priest
Ok, my theory is father Thomas has sin in him and has never heard the "calling"
Some hints to this would include the sister telling him he can quit and don't talk to a married woman. Then the fundraiser father says he's the next big thing in the Catholic Church.
So does the devil/demon want corrupt his soul and take out the only priest willing to fight him at the same time..?
Never seen the movie and not sure if this is a reboot or not, but that's my guess. Pretty good pilot though