I do think that only an authority can tell you what the right belief is.
Maybe. I think that I'd prefer to think of it a little differently -- An authority can help us reach the place where we're able to recognize for ourselves what the right belief is.
But this is so only in principle; in spiritual/religious practice, it is a dead end since the run-of-the-mill person is per definition disqualified from recognizing authority.
I'm not sure that I totally agree with that one.
Certainly I do agree with what you said in an all-or-nothing sense, where we are talking about attaining THE ULTIMATE. Which obviously begs the question whether it even exists and is accessable to human beings.
Of course, if "run of the mill" (presumably meaning 'normal' or 'finite' or 'mortal') people are incapable of penetrating through to knowledge of transcendent things, then that would seem to once again be a pretty good (and rather familiar) argument for agnosticism regarding transcendental matters. If we take that line of argument seriously, then it seemingly applies to all of the purported religious authorities right along with all of the rest of us.
Where I think that I am going to disagree with you is in my intuitive feeling that incremental progress is possible on spiritual paths. Progress doesn't necessarily suggest that any of our teachers has reached some transcendntal apotheosis, but only that they have deepened their practice a little beyond where we are at the moment, and that they might conceivably have something worth teaching us.
I'm just naturally doubtful of suggestions that some people (your 'theists'?) have some magical pipeline to transcendence, or even if some of them somehow do, that any of the rest of us could ever know it. I'm equally skeptical of the Indian tantric traditions of guru-worship.
I much prefer the idea of everyone being peers on the path, being students of spirituality, fellow seekers, or what the suttas call 'spiritual friends'. Some of those around us might well be more knowledgeble than we are, more compassionate, more proficient, or less self-involved. But that can't just be proclaimed, it needs to be demonstrated. It grows out of our deepening knowledge of and respect for the truly authoritative ones and it probably isn't ever the sort of unbridgeable transcendental chasm that you are imagining.
Our teachers. the "authorities", are people just like us. It just happens that they have something valuable that they can teach us in a particular instance. Maybe in another instance we will be the ones be teaching them, or perhaps both of us will find ourselves alongside one another learning together.
You know, not only the sangha, but university graduate school illustrates this. Students progress out of undergraduate classrooms (learn the material, take the exam) to situations where they end up embedded in their professors' research teams, working together to make progress on a scholarly problem that none of them entirely understands.