The first definition from a Googling...
An idiosyncratic belief or impression that is firmly maintained despite being contradicted by what is generally accepted as reality.
This is Mormons. It is that simple. They do not hold to what is generally accepted as reality.
The problem you have is that, the more broad (or narrow) your sampling of culture, the more the "accepted reality" changes.
So, how do we measure what is true that applies cross-culturally? Empiricism. The problem with empiricism is that it, essentially, proves everyone wrong. It has no friends amongst people who are desperately clinging to their culture. Atheists are not immune, but, certainly, they are more inoculated.
If you want to know the truth about your own personal delusions, then you are going to have to transcend your own personal culture. In order to do that, you need to be psychologically vulnerable in ways that people are not usually comfortable with. In reality, this is why people have such trouble with learning about their own delusions; people aren't normally interested in the truth, wherever it may lead them. People are more interested in being comfortable and unchallenged.
We all have blind spots in our perception and I'm not just talking about sight, taste, touch and smell. Our unconscious processes work behind the scenes to make us act and it is only afterwards that we apply rationality to it. We do this using the lens of our own personal culture. It is the same way we apply context to our dreams. We wake up, view our experience through the lens, then change the details so it makes more sense to us. Everyone does it. Even while it is happening, we are trying to build a cohesive reality out of random neuronal firings.
If you are going to try to defend against Mormons being delusional, you can't win. No one can win, because we all maintain delusions that help us operate better in a culture with a primitive way of thinking. Whatever you would like to think about your capacity for reason, you are still an animal with instincts that control your behavior on an unconscious level. We all participate in the great competition for resources and mates, and whatever you think you are doing, that is what your true motivations are all about. It is even why you think the world works the way you think it works. And, of course, you are wrong about that.
Everyone is wrong. Everyone is delusional.
I think it is safe to say that Mormons are farther down the spectrum than most groups, but not as far down as paranoid schizophrenics. Why? Because they cling to beliefs that are clearly made up fantasy.