There is so much stigma around mental disorders. In my graduate studies I took a survey of people's perceptions of "what is mental illness" and I got reactions from "crazy people" to "people who scream things out at you as you walk into a store." People I surveyed said that people with mental illness could be sociopathic, unable to control themselves, and be a danger to others because of the nature of their illness (i.e. they might leave the house with candles burning or drive a care without a license, etc.) The ironic thing is that many of the people I surveyed were on medications for things like anxiety, depression and ADHD.
The thing that makes mental illness different from physical illness is that mental illness is often a result of imbalanced neurotransmitters (brain chemicals), but it also is affected by experiences (war, abuse, trauma, etc.) biology (there is scientific proof demonstrating that some people are more prone to develop certain disorders), relationships (does the person have a strong support network or are they left to deal with life on their own), upbringing (did they have a healthy, stimulating environment as a child or were they neglected and abused?) etc.
To say that a mental disorder is all spiritual or all biological, I think, is too simple of an explanation. Satan knows our weaknesses and is ready to exploit them every chance he gets. If he knows a person is prone to depression, don't you think he will use every opportunity to exploit that? Chronic pain? Satan is ready to use every weakness to his advantage, and susceptibility to addictions or schizophrenia is definitely a weakness and he often knows all the right buttons to push. On the other hand, it is just as easy to say that Satan can cause physical afflictions as well--headaches, chronic pain, etc. Does he really CAUSE it or does he set up circumstances for it to happen, or both?
On the other hand, it is unloving and unhelpful to tell people who suffer from mental illness that they just need to pray more and believe God will heal them. Would you tell that to someone who has cancer? Medications can do wonders in a person's life, and God has allowed them to human beings as a gift, both for physical illness as well as mental illness..
I hope that offers a little insight.
Be blessed