You’re coming at this from a good place Cristina, but I have to disagree with you; and I say that as someone with a mental illness (depression).
Religion absolutely is a mental illness. Specifically I would say it’s a delusional disorder, which I would define (as a syncretic definition taken from several medical and psychological dictionaries) thusly:
A fixed false belief held without and/or against evidence.
There’s objections to this, some of which you have mentioned yourself, others of which I hear quite often.
1. Religion is Excluded from the Definition
It is true that many of the official definitions of religion specifically exclude religious or cultural beliefs that are widespread. The alternative term ‘relusion’ has been suggested, but this wouldn’t include ideological or political indoctrination in places like North Korea, so I don’t think that’s a solution. Excluding religion is such an egregious case of special pleading that I don’t really know why anyone would do so, other than a residual unease at criticising religion.
Yes, a lot of people are religious but when people fall sick we don’t change the definition of healthy, even when it’s a virulent pandemic. Do the beliefs fit the definition? Yes? Then that person is mentally ill/bonkers/insane. Simple.
2. It’s a Choice
Is it? I don’t know that I chose to be an atheist, it was the inevitable conclusion given logic, reason and evidence. Choosing to ignore reality for a comforting myth is a form of mental illness of its own (Freudian denial), is it not? Do kids have a choice? No. Do adults? Sometimes. However we acknowledge that it is possible to give people other forms of mental illness. We can torture people to break their minds, leave people with PTSD and anxiety and otherwise screw them up and leave them mentally harmed without their consent so why should religious indoctrination be any different? With mental illness we have some choice. We can take the meds, do the mental exercises, take up CBT, undergo therapy all to minimise or even cure our mental issues or at least learn to deal with them. So there’s an element of choice there too. Doesn’t invalidate that they’re still mental illnesses.
3. That it’s Insulting
In the words of the prophet:
Is it accurate? Yes it is. That’s all that really matters. I’ve tried to make this point to people who use the term ‘cisgender’ whilst apply their own subjective standard to other correct terms and trying to get people to stop using them, but it doesn’t seem to ever sink in.
If your doctor tells you that you have cancer, he’s not insulting you. If someone with sufficient knowledge tells you that you have depression or are delusional, it’s not necessarily an insult either. It’s a diagnosis.
4. It Minimises the Seriousness of Mental Illness
Does it? I think it brings home the seriousness of it as regards religion. While mental health issues still aren’t taken as seriously as, perhaps, they should things have improved a lot in recent years. People have a better (but dysfunctional) understanding of what it means to be mentally ill and how debilitating it can be. Pointing out that religion is a mental illness helps, in my opinion, lend weight to our criticism of it.
There is one big, important, terrifying difference between religion and other mental illnesses though.
Religion is communicable.