please note: everything in this post is my opinion. I am not a doctor.
When I was a christian, there was one thing that I felt a lot: guilt. Christianity is about guilt and pardon (or parole, in some sects). Also, a belief has arisen in demons, demon possession, demon oppression and measures one can use to banish them. Then there is religious experience. For example, feeling that you are in a higher state of consciousness as you sing repetitive religious songs with a crowd of other believers. There is also the idea that you have received some special insight from god, the idea to pray for someone or even marry someone!
If you have an undiagnosed (or even diagnosed) mental illness, these concepts can really get out of control.
I felt so very guilty, Jesus had to die terribly for me after all. However, since I also suffered from major depression (the official diagnosis), excessive guilt happened to be a prominent symptom. See how these two things dovetail into a hellish frame of mind?
I am not schizophrenic or psychotic but I remember meeting a nice young man at church. Let's call him Doug. Doug had major demonic experiences. They appeared to him and spoke to him. We well-meaning young christians prayed for him intensively, advised him on items to remove from his home that may be attracting the demons. We meant well but that wasn't good enough.
I see now that he needed medical help and should have been advised to go to a psychiatrist. I deeply regret that.
Later on, I was diagnosed as bipolar 2 (the milder form). With bipolar disorder, a person can get grandiose ideas in his head(during the manic phase) and interpret them as truth. This can come with a feeling of euphoria. So I have a "revelation" about something related to god or some religious experience that seems so true. I am encouraged in these beliefs by other christians. There's another dovetail from hell (which I don't believe in, haha).
This next example is written in a spirit of deep wounding and anger: most christians are terrible about responding to depression. They tend to see it as a lack of faith, deliberate wrong-thinking or spiritual deficiency. Worse, they can decide that you are oppressed by the devil. Oh shit!
I have been in situations with this kind of ignorance when reaching out while depressed to the point that I kept my depressions secret. And if you hide a part of yourself, how can you get truly close to others. Because we are supposed to "fellowship" with other christians, no?
Fortunately for me, I had a friend in the 12-Step program who was very practical and said "Maybe you should see a doctor." Not all are so fortunate.