These are beliefs that are firmly held, but false, and out of keeping with normally-accepted cultural norms (religious beliefs would not count as delusions as they are culturally 'congruent').

They are not uncommonly a symptom of Psychosis, which will require a swift response.

If the Delusion involves another person, and particularly if it involves ideas of Violence or is particularly Paranoid in nature (even if it is a delusional belief that "X loves me!"), then it is a risk factor for aggression that should be taken seriously (see Managing Risk).