I don't think the term "validity" is really meaningful in this context. Perhaps factual is more appropriate to your argument.

And you can play the numbers game both ways, too, of course:
If only one person disbelieves a set of spiritual beliefs, they are certifiably insane. etc. etc.

The entire argument of whether or not a religion is "valid" or "true" is mostly irrelevant. When billions of people believe something, they will act as if it is factually true and whether or not you agree with them, you have to take that into consideration. I accept that many people believe things that I do not, and I moderate my behavior according to my level of vulnerability as a minority.

If I'm unarmed and everyone around me carries a spear and believes that the trees are the residence of angry gods, I smile politely and walk around the trees too. At times, living in the southern area of this supposedly democratic society feels like living among armed tree-god-worshippers, so I'm careful in which surroundings I make my non-default beliefs obvious. Most of the time, quite fortunately, it's simply not an issue.

So the point is that certifiable insanity of an individual is differentiated by society from ordinariness simply by a purely arbitrary count of those who share their theistic worldview. Beliefs based on evidence employ a peer review process as well, however it works very differently.

I don't believe this works in the other direction, so I reject that as a false equivalency. It is absurd to imagine a million people supporting a faith-based religion with only one individual being skeptical of it. That's too fantastical even for the Twilight Zone.