Most of our notions of mental illness are historically quite new. Only in the last century have we humans tried to understand ourselves. In past centuries only the most obviously mentally ill were identified as such. During much of human history even those who were blatantly mentally ill were not recognized as such. No effort was made to segregate them from the rest of society much less treat them. Thus, much of what should have been seen as insanity was instead simply accepted. Even now, human understanding of the limitations of the human mind is in its infancy, and most mental health professionals are doing little more than guessing as to what is an illness and what is healthy.
For the most part, this guesswork is premised on the assumption that whatever helps the individual get along with others and get through life is healthy. As a result diagnostic criteria contain exceptions for things that are believed or accepted by the majority of people. The definitions are not premised on notions of objective reality.
If a modern mental health practitioner treats a patient who thinks he is immortal, thinks he has an invisible friend with magic powers, thinks that his invisible friend also happens to be the most powerful being in the universe, describes his invisible friend as being brutal, controlling, egotistical and demanding, and says his invisible friend will "forgive" the patient for any transgression he commits without punishment or compensation to the victims, the practitioner would undoubtedly diagnose the patient as being dangerously insane. If the patient simply comes in to the mental health practitioner and says he is a devout religious person, the practitioner will think nothing of it, even though he has essentially said the same thing as the patient who was dangerously insane.
You can shorten this point a bit and use it whenever the believers ask something like "Do you think believers are just delusional?"
You can reply with some version of this:
"You think you are immortal and that you have an invisible friend with magic powers."
If necessary, you can point out that this invisible friend also seems to be crazy ("Worship me or I will torture you forever.") and that in the case of most people who have invisible friends, the invisible friend is simply a reflection of the person's inner self.
This point can also be used when arguing against those who say that religion is a good thing. Part of the reason it is not a good thing is that it undermines the believer's ability to think rationally about both morality and reality.