Most scholars freely concede the unusual and perhaps even abnormal aspects of Luther's personality, without accepting the diagnosis that attributes these traits to an underlying psychosis. By most standards, Luther was a neurotic man who, later in life, suffered from bouts of depression. Given all the evidence of productivity, clarity of thought, and ability to work with others, however, it is highly doubtful that he can be properly diagnosed as a psychotic.
2) Preserved Smith, The Life and Letters of Martin Luther (Boston and New York: Houghton Mifflin Co., 2nd edition, 1914 ), Preface to the Second Edition, p. viii:
Of Luther's early life and development prior to 1517 I have now arrived at a somewhat different conception from that set forth in the present biography. Sturdy as was the Saxon's constitution, a neurotic vein may be detected in his violence of language, in his obsession by the devil, and, one is tempted to add, in that conception of God as a cruel and capricious tyrant, which he himself confessed was repugnant to natural feeling.
3) R. C. Sproul, The Holiness of God (Carol Stream, Illinois: Tyndale House Publishers, revised edition of 1997; paperback edition of 2000), p. 78:
Luther's chronic stomach troubles have also been linked to a psychosomatic problem. His neurotic phobias all seemed to go directly to his stomach, destroying his digestion.
4) Michael A. Mullett, Martin Luther (Florence, Kentucky: Routledge, 2004), p. 101:
. . . the kind of extreme scrupulosity that Luther himself had known over trivial offenses . . .
[Overscruposity is considered a neurosis]
[written in reply to a Catholic who thinks this]
Neurosis is a different thing. We know that Luther was overscrupulous, from the reports of his ridiculously detailed confessions as a Catholic monk. Talking to the devil doesn't necessarily prove anything. Jesus did that. I'm sure many saints have as well. It could possibly be a weird thing, but not necessarily.
I don't see the benefit of arguing that he was nuts. It is only seen as a personal attack. I prefer to stick to his false theology and ideas. That's where the heart of the discussion lies. As for this other stuff, I go with what almost all Protestant biographers believe: he suffered from periodic serious depression and had some other eccentricities.
We need not regard a man as either evil or insincere (let alone nuts or demon-possessed) because he taught some false theology. He was still a Christian.