Let's say that someone with DID (dissociative identity disorder) has 2 alters: an atheist and a faithful Christian. Where would that person go? To Heaven? To Hell? To some third place?
This question is pretty important as it is estimated that 1.5% of the global population (or 121 million people) suffer from DID, many of whom are faithful Christians.
From God's point of view, there is only one soul. Consider Edward who suffers an advanced Alzheimer's disease that causes him not to recognize his own wife & children. Would his family disown him? Of course not.
But let's say Edward is healed from his condition and is shown video recordings of the occasions where he didn't recognize his family. Would the healed Edward say that he was being himself? Of course not.
So a simple answer is by asking this question: which of the 2 alters is primary? Would the healed DID say "I'm truly a Christian" or "I'm truly an Atheist"? I forgot how the Dr. Jekyll & Mr. Hyde story go: did Dr. Jekyll apologize and make amends for what Mr. Hyde did? If so, then the SINGLE human soul operating alternately in BOTH Dr. Jekyll & Mr. Hyde two psyches can choose that Dr. Jekyll is the primary. A similar evaluation can then be applied to the DID Christian case, where the Christian's soul when operating in his Christian alter ego will then invoke Jesus pleading that he will be healed from this atheistic alter ego, believing with St. Paul who said in his letter to the Romans:
For I am persuaded that neither death nor life, nor angels nor rulers, nor things present nor things to come, nor powers, nor height nor depth, nor any other created thing will be able to separate us from the love of God that is in Christ Jesus our Lord. (Rom 8:38-39)
Prior to the Day of Judgment everyone will be healed from any conditions that could affect one's decision for God: whether it's possession by Satan, DID, Alzheimer, traumatic childhood causing one not to recognize God's love, misunderstanding of the gospel rooted in how it was presented in a corrupt form, etc. Everyone would be able to see Jesus for who He really is: a loving God in human form who wants to have a love relationship with you in a way that purifies your own love for God if there is a kernel in you willing to be united to God. (For more on the centrality of this decision even though the process may not be completely transparent to us, hear Eleonore Stump explaining How Does Salvation work? especially starting at minute 7:16 which immediately precedes the case study of a "psychological disability" and another case study that makes the decision for God obscured from our normal psyche starting at minute 9:00.)
In one's most lucid moment one is either FOR or AGAINST God. There is no gray area / middle ground. C.S. Lewis famously said in his book The Great Divorce (bold from me):
“There are only two kinds of people in the end: those who say to God, "Thy will be done," and those to whom God says, in the end, "Thy will be done." All that are in Hell, choose it. Without that self-choice there could be no Hell. No soul that seriously and constantly desires joy will ever miss it. Those who seek find. Those who knock it is opened.”
And that decision is what God honors and He will give grace to overcome any physical, mental, or emotional condition that is part of humankind suffering in general, and to purify & perfect a soul's inability to love 100%.
This question makes no sense, because you are viewing the situation in a legal sense. (and consider heaven and hell as places instead of states of being)
If you stick to the legal sense that your original question uses, then I would simply point out that "insanity defense" is legit in modern legal systems and why would God be less merciful than us humans?
However, I will actually stick to the fundamental frame issue as it is more useful for you and other readers.
From the Eastern Orthodox perspective questions like this are "non-questions" because God is Love. As I am not an "infernalist", I believe that God truly does not want anyone to perish. I believe that the fires prepared for satan and his angels are the purifying fires of love. There will be Apokatastasis.
This position is one that is not clearly defined within the Eastern Orthodox Church and I am only presenting the side I personally believe. Both "inclusivist and exclusivist tendencies have characterized Orthodox theology for hundreds of years. However, I personally find the exclusivist tendencies to have a "god that is too small". -Wyrsa
Within the many ikons of the Last Judgment they depict Christ surrounded by all people and angels, seated in glory and light with books opened by his angels as well as the demons thrown into the eternal fire. The scene is not a “threat” to be good, but a revelatory reality that Christ has been given the authority to judge by God the Father. This teaching is not a fear tactic to manipulate us into being good. Mercy flows out of the heart, not a coercive act of kindness.
John Chrysostom teaches that “our thoughts will stand forth…[to] condemn … exonerate us.” Christ is the judge, but not the condemner. He reveals each person’s nous, their heart, and He reveals humanity.
The Orthodox church teaches that all have been given enough of their own judgment to decide what is right or wrong. So for Jews, they will be held to the words of the Law. For Gentiles, they will be held to their inner law, what we call a conscience, or inner knowledge. Christians are judged by the Gospel of Christ. All of these moral criteria come from the Most Holy Trinity, not merely a theory of natural law.
All cultures follow a way of behaving, a code of morals that have been given to understand God the Creator in our actions toward others, no matter what level of light is given to our minds. Whether by Law, Conscience or Christ, the soul is assumed to exist. Basil the Great teaches that our bodily actions become imprinted on our soul like a painting, and that we are all judged by people who lived in similar situations and positions as we did, since cultures vary, and we are born into different times and places. There is no room for rebuttal of unfairness. Even the mentally ill will be judged, but they will be judged fairly.
No one is deprived of God’s love after death, even in Gehenna, teaches Isaac the Syrian in his Ascetical Homily 18. That God wills universal salvation of each person is scriptural (For God does not wish for any to perish...)
Protopresbyter Georges Florovsky teaches that there is only one possibility: all will experience God’s eternal, unchanging love.
Short Answer: Mental illness doesn't damn us to the state of hell. God isn't waiting to leap out from behind a bush and say, "Gotcha!" he is standing openly with his arms wide for everyone. Mental illness is no different than any other illness. A schizophrenic isn't responsible for their hallucinations and delusions any more than an Alzheimer's patient is responsible for their memory loss. Mental illness is one of the crosses some of us have to bear. God loves and understands.
The judgment of God does not remain on people who do not know what they are doing, remember this famous quote by Jesus? Father forgive them, for the know not what they do, the same stands for crazy people and other men suffering from diseases that make them lose touch with reality.
Luke 23:34
Father, forgive them, for they do not know what they are doing.
This verse automatically justifies crazy people and people who are not aware of what they are doing, but to those who know what they are doing then they will be required to account, these crazy people have that forgiveness and will go to heaven because not even their craziness can separate them from the love of God through Jesus Christ.
As Traditional Catholic Angelic theology and history shows that even those advanced towards God, as later they recognized as Saints, may be Demon Oppressed or Possessed, and all such Oppressions and Possessions are Willed by God to exercise and increased those souls, it may be assumed that death with such a Cross - His Judgement would not be prejudiced negatively.
Since a significant percentage of all Gospel-documented Ministry of Jesus delt directly with Demons I find it incredible that with such a question assumes the Modernists framing of shallow and ungrounded phycology for what is most often Domonic Oppression or Possession caused (except for the rare few that have brain lesions, for example) and routinely cured in most of Church history by exorcism, but instead assumes the framing of 'Mental Illness' that directs courses of treatment that fail, and so traps the victim in lifelong suffering.
The Traditional Catholic doctrines of Angelic Theology are rich and are sadly mostly unknown by laity and even by most Ordained in recent times. As such can be argued this ignorance is one of the early successful attacks by Modernists on the Church.
Consider the tens of billions of people throughout history that never even heard of the God of the Bible, much less of Jesus (the only name by which one can be saved). Their situation is not really any different from people with mental disorders that leave them incapable of receiving salvation, or children that can't comprehend what salvation even means.
An answer to this specific DID question will almost certainly provide an answer for those people too.
But the question itself needs to restrict answers to a specific denomination.
For those denominations that believe in "soul-sleep" and redemption after the second general resurrection (excludes most Catholic-based denominations (e.g. those that believe in immortal souls going to Heaven at death)), the answer is trivial:
So one simple answer is that people with mental disorders that render them incapable of accepting salvation will not be called during this age. They will die unsaved, they will remain in the grave (in old English "hell"), and then they will be resurrected at the end of the Millennium, cured of their disorders, taught God's way of life, and finally offered salvation.
For more details, refer to my answer to How can there be mortals in the millennial kingdom
