"Believers" is an "ok" episode for me. I like that the resolution of the plot isn't a "pretty" one; there's no easy answer.
I liked the series of scenes of the parents going from one ambassador to another trying to get someone to speak for them and how none of them would jump onboard. Kosh's line "The avalanche has already started; it is too late for the pebbles to vote," while being typically Vorlon, I think it also hints at perhaps some underlying regret or similar emotion from Kosh over his own instance of requiring medical intervention against the wishes of others.
It was a very good episode for Franklin and gave a lot for Rick to chew on as an actor.
What keeps me from viewing this episode as stellar though has a lot to do with finding the acting of the Shon character to be rather weak. Also, the that the religion of the aliens feels just sort of slapped together to suit a one-use need for this episode. I'd imagine there are plenty of people who disagree with me on this issue of their religion, but it's how it seems for me. I guess for so much of this episode's plot being reliant upon the "alienness" of the aliens, it just didn't feel alien enough for me. It might just be a side-effect of a one-shot alien culture since it's something that's gotten me about a whole lot of aliens in various episodes of Star Trek too.
So, for me, it's balances out to an average episode.