Definition of "delusion" in DSM-5

[Rev. Rye]

It has to do with the crippling effect of the delusion. If you believe Elvis is alive and you pay your bills go to work, school,etc. then you are just irrational and misguided. If you will not leave the house because you think Elvis is showing up to marry you (true story with a family member of mine) then your thoughts are delusional. When I worked as a psych nurse we tried to teach patients how to distinguish between their "normal" religious thoughts and those that are signalling another psychotic break. Religion was never discouraged and I know why. My entire family is religious, I have cared for those in a a psychotic delusional state, and these states are different.

Surely you're changing the definition here. The DSM-5 doesn't say anything about the crippling effect of the delusion.

I took ab-psych a couple years ago: it's generally assumed that diagnoses should really apply only if the symptoms cause a marked impairment in daily functioning. I once asked my teacher about Elwood Dowd, Jimmy Stewart's character in Harvey, who seems to live a normal life in spite of his firm conviction that his best friend is a 6 1/2 foot tall rabbit named Harvey. I asked if there would be a point in diagnosing him with anything in real life, and the teacher said probably not.

Looking at my own copy of the DSM-IV TR (I found a copy in the clearance rack at a Half-Price Books a year ago and keep it around), there doesn't actually seem to be much in the definition in the OP that isn't in the DSM-IV TR. In the relevant section (page 326):

An individual's cultural and religious background must be taken into account in evaluating the possible presence of Delusional Disorder. Some cultures have widely held and culturally sanctioned beliefs that might be considered delusional in other cultures. The content of delusions also varies in different cultures and subcultures.

And, of course, there are legitimate reasons to criticise the DSM-5 (most gallingly their gutting the section on autism spectrum disorders and eliminating Asperger's entirely), but I really don't think this is one of them.