At the risk of repeating myself I'll weigh in here: X is an authority with respect to a proposition P to the extent that X's assertion of P is evidence for P.

On many topics, some people's assertions are stronger evidence than others. That makes those people authorities on those topics, relatively speaking.

To my mind, the interesting question is how we best distinguish actual authorities on a topic from people who merely claim authority. That's difficult. But the first step in learning distinguish among A and B is to acknowledge that A and B actually are different things: in this case, that actual authorities on a topic are a distinct thing in the world from non-authorities.

Asserting that there are no authorities, or that everyone is equally authoritative, is a step in the wrong direction.