Forgive me for being confused, but I always thought agnostics were undecided if there's a God or not. I've known a few agnostics and that was how they described their beliefs. None of them said "I can't know" if there is a God or any sort of higher being.
Don't worry, I'm confused myself. I overshot a bit.
Not all agnostics think that whether or not deities exist cannot be known. Some just think we currently don't have enough evidence.
However, the "not knowing" isn't simply personal. If they just personally didn't know, for example, they could think like this: "I don't know whether God exists or not, but the priest knows, and he's an expert in spiritual matters, so I'm going to believe, too."
In normal conversations, it's impolite to corner people like that, so agnostics won't push the matter, and you - the believer - won't tell them to believe the priest because he's an expert and knows better.
Think of it like this:
- I personally have never seen a platypus. Thus I don't know if the platypus exists. There are people who have seen one, though. There are pictures, etc. Thus I think there is evidence that the platypus exists. I'm not agnostic about the existence of the platypus. I know it's there.
- I personally have never seen aliens on earth. I do not know whether aliens have visited earth. There are witness accounts, pictures, etc. But I do not accept them as conclusive evidence. I think that we do not currently have sufficient evidence to say whether aliens have visited earth or not. So I'm agnostic about aliens having witnessed earth, but only "temporarily", because I think that it's principally possible to discover evidence for aliens on earth.
- I personally have never witnessed God. Lots of people claim to have such experiences, but they're non-conclusive to me. I believe that we do not have any evidence for God. And - so far - I'm agnostic about God in exactly the way I am about aliens having visited earth. But, unlike with aliens, I don't think there can be evidence for God. The arguments for that are pretty complex and it's not like I've thought them through to the end. But I do think that if there is such a thing as a God, humans are not equipped to understand the concept. Proving or disproving god is a waste of time, and so is believing.
Likely, your friends don't go as far as I do, and they stop at the "aliens level". But I'm pretty sure they'v passed the "platypus level". That is, they not only don't personally know whether God exists or not, but they think the knowledge is of a type that doesn't let them accept the priest's authority.
Sorry for being confusing. I hope this helps you through the mess a bit more.