First of all, I've known people who've gone into and out of religion for all kinds of reasons.
The issue is what Neelix believes. If Neelix has always believed (even taken for granted) that he will have a spiritual near-death experience, should he ever have a near-death experience at all, then the absence of that experience is faith shattering to him. Someone else who believes in something else can say that Neelix shouldn't have believed he'd have the experience or that he'd understand the experience, that Neelix isn't very religious at all if his religion is predicated on something like that. It doesn't matter to Neelix.