The opposite of a Christian is not an atheist, it is someone with mental illness.
Christianity posits that you can trust your inner voice, mental illness demonstrates that you can’t. People say “God” tells you things that are outside the scope of what you could come up with with rational thought alone, but so does mental illness. And it does so at a moment of weakness where you won’t be able to differentiate between the two (if there was even a way to tell in the first place). As for me the voice of God told me to drink urine, kill myself, and date women out of my league. Suffice it to say I stopped taking it seriously rather soon. Now I get it that people without mental illness and even people with mental illness can trust their inner voice and I’m happy for them but that doesn’t mean this is universal.
Mental illness often hinders your ability to project the idea you are actually a Christian to those around you. Lets be honest, Christianity is performance art and part of being successful at it is selling the idea you are in the faith to your peers. When your Christian peers stop believing you believe, even if you still believe, it drags it down as faith is more of communal property than our individualistic culture would like to admit. It is very hard to perform when you are dying inside. Some people put on a show while struggling mentally but eventually they just break down after being pushed to the limit and sometimes end up completely different people. That didn’t quite happen to me because I was never good at putting on a show but many people are because they like the affirmation a community offers in exchange for their performance.
People with mental illnesses often don’t “experience God” or if they do it’s toxic. This ties into the first point. A lot of Christians brought up in the church who struggle with depression pine for an experience of God that is promised but never comes. In churches (particularly ones that feature praise and worship) God is sold as an entity that you can have an emotional connection with. The problem is with mental illness the ability to “feel God’s presence” is impaired. But depressed people pine for this experience repeatedly and it just makes them cycle down. In some denominations despair is a sin but it is also what a depressed person feels routinely, so it’s tantamount to making the symptom of a disease a sin.
Those with mental illnesses’ lives rarely go according to “God’s plan”. “God’s plan” is a loaded term which means a person is promised a life of material and social prosperity or at minimum a dignified existence with troubles that are manageable. It’s no secret our American culture is obsessed with success, health, independence, money, and status. When you don’t possess these things you are treated worse by Christians and non Christians alike. Christians want people who suffer to concoct some kind of redemptive narrative so they don’t have to be exposed to senseless evil and chaos. For me losing one thing after another to mental illness there was an incredible amount of shame dealing with Christian peers.
Flat affect and negative symptoms challenge the notion of the soul. Some people with severe mental illnesses lose so much of them selves that it’s almost like some of them has died. This makes people but especially Christians uncomfortable because they have a dualistic view of humans that there is a soul that is impervious to condition of the anatomy. Modern science says the self resides in the brain and seeing severely mentally ill people this is shown to be true.
People with mental illnesses are often ostracized by the church. I was. It’s one thing to call someone a liar, it’s a whole other thing to intimate they are one with every thread of your being. The reasons I have listed above make Christians very uncomfortable with people with mental illnesses. For those who hold tightly to certain beliefs cognitive dissonance is experienced as psychic pain. On a more practical level people with mental illnesses often say things that are inappropriate or only have conversations that are suitable for one on one interaction as they don’t have much to make small talk over.
What you can do to help. Believe people with mental illness when they tell you how bad it is. Offer material support like rides and inclusion, you generally have to build rapport with people on the margins before you will make much headway with them ministering.