I have a friend whose theory is that cats are able to turn gravity up and down at will. Thus their ability to either sneak very very quietly to catch mices or go GALUMPH GALUMPH down the hallway like a thousand charging elephants, as the whim takes them.
He does own four of them though, so I think we can excuse him for his theories 
Seriously, though, I think where a lot of confusion is coming from in this discussion is that the OP conflated two cases which, IMO should be kept completely separate.
Case 1) All the data on {whatever the wierd opinion is about } is freely available to everyone, and everyone draws logical conclusions based on the agreed data.
This encompasses people like those who think that there was a CIA conspiracy to kill JFK, or the American government is hiding aliens at Roswell.
In these sort of cases, we tend to be less likely to view people with “out there” views as complete loons if there are lots of people who think the same way, even if we ourselves are in violent disagreement with their position. This is because we recognise that our own logic can be fallible, and so when we disagree with someone on the state of reality it might be us who’s mistaken
Case 2) Some of the data on {whatever } is only available to the person whose sanity we are questioning.
This applies to people who hear voices, or have memories of aliens anal probing them, or whatever.
In this case we are MORE likely to judge someone a loon if there are lots of other people saying roughly the same thing, particularly if the claims of all such people are similar, but conflicting (eg - three people all claiming to be Jesus)
Back to the OP - it was discussing a guy called James Matthews who thought he was being mind controlled by Some Vast Conspiracy. And the point was - why do we judge him insane given that lots of other people also think they’re being mind controlled. That is, we’re thinking of Matthews as a “type 1” under my classification scheme above.
My contention is - he’s actually a “type 2” because I’ll hazard a WAG that the specific things which caused him to believe in the conspiracy were things which only he could detect (ie ‘voices’, strange-feeling impulses and so on). Therefore the mere fact that there are other people with similar sounding opinions should NOT lead us to conclude that he’s sane.
In fact, concluding that Matthews was sane will lead to big logical problems because we should then conclude that all the other people who talk about Vast Mind Control Conspiracies are also sane, and in fact there is one. And I’m guessing that all the various Mind Control Victims out there CONFLICT with each other on the details of this conspiracy, so that we can’t believe them all at once.
If they didn’t conflict with each other, (for instance if all the Mind Control Victims independently agreed that the mind control was by microwave and that their base was under the icecaps in the North Pole) then in fact we would be justified in judging them sane and sending an excavation team to Santa’s House.
Thinking about this, perhaps I should extend my classification scheme to three cases:
Case 1) Data is publicly available to everyone to draw their own conclusions from. Data doesn’t conflict (obviously, since we can all see it ). No insanity here.
Case 2) Data is only available to the affected people. Does conflict. Affected people probably insane.
Case 3) Data is only available to the affected people, but they all or most (without collusion) agree on a non-conflicting interpretation. Affected people probably not insane.
Now I would argue that we can make a good case for at least some of the “charismatic christian” stuff falling into case 3 rather than case 2.
But perhaps I should leave that for another post, since if I make this any longer the Great SDMB Post-Eating Monster will probably target me…