Some Christians. I am not really the person to defend people whose only defense against the DSM IV definition of delusion is that they are explicitly exempt from it (because else any religion very much fits the definition perfectly), but it should be said that not all of them are THAT delusional. Only a rather tiny minority, and close to 100% of that minority residing in the USA, actually believes that.
That definition was changed in DSM-V. Significantly. The delusion no longer has to be demonstrably false. Now, they can believe that it is true, and still be diagnosed with a mental illness, if their behavior warrants it. But it also means that it is up to the clinician making the diagnosis. Parents who sincerely think praying is going to heal their child should not be penalized for holding that belief if the clinician determines there is no danger to the child. As long as their delusions are doing no harm in the opinion of the clinician, they fit the exemption. But it will be harder to ask for an insanity defense -- they will have to face their crime for what it is if the child dies.
Outside the US, new earth rubbish plays no significant role.
Ahh, yes. But it does play a significant role in the U.S. It is perhaps unlikely, but it is certainly possible, especially if any one of the current GOP candidates for president actually win, that somebody that (emphasis yours) delusional could achieve the highest office in the land. You really, really don't want an American president, a man who can call down a nuclear strike if he thinks it is necessary, to believe the earth is only 6000 years old, and to believe there is an invisible man in the sky telling him to do it... .