Oh sorry, I misunderstood.
In this case my explaination would be:
Although Avatar's world has lots made-up things, the general life of that world is pretty much the same like where we live. I mean, although the creature are totolly different, their chef are fully comfident at cooking them; the virus maybe difference too but doctors in there know how to treat it. so in case of pastor, I believe they also clearly know their duty, even we can't us our logic to explain their faith.