What about when your using "offensively", against other realms? Say you want to reduce the morale and loyalty in a region of an enemy realm with their own religion? High oratory skill would be a necessity here to get maximum effect. I've never really experienced the priest game and seen a blown out religious war between priests.
Two Priests in an enemy region can be very effective, but one Priest can pull things off too. Population will of course greatly matter (Smaller regions are easier. Armies can very much help you reduce the amount of peasants to convert too) A Lord in the region holding courts will make things more difficult too. But when you got the region converted and are left alone for a bit, you can quickly destabilize it by scaring the population or causing unrest, opening up other options. Decreasing loyalty is trickier, as it makes you lose more followers, and will probably cause a reaction by the Lord / Judge. Increasing Sympathy can be done if you're given some time, but you should really ask yourself whether it's easier to use an Ambassador or a Priest when dealing with loyalty/sympathy. It depends on the situation. As a Priest, you basically poke the enemy until you find the weak spot, then exploit it.
As an example, Ibladesh has been poking and found and exploited a weak spot by taking Bursa (A small mountain region) from Caligus. It's now rogue, but Ibladesh has denied Caligus the use of the region, and it will take a while if Caligus wants to get it back under control. Ibladesh has managed to convert various border regions, which will make arrests of Priests more difficult, and allow Priests to start harassing as soon as they can use a weak spot.
Coordination and working together can be critical in offensive use of Priests, and the Priests do require to have high oratory skill if the enemy will attempt to take action against them.
