In 1670, the astonishing confession of Major Thomas Weir suggested the depths of depravity that could be concealed under a cloak of holiness and respectability. Throughout his 70 years, he had been a paragon of Edinburgh's rigorously orthodox and moral Calvinist society, and had earned glory in the Lord's wars of the 1640s. Suddenly, he spontaneously confessed to a litany of crimes that included devil-worship and witchcraft. He admitted to incest with his sister, herself an exemplary model of righteousness, and she confirmed the charge. Both were executed. Weir rejected pleas that he pray for forgiveness, declaring that "I have lived as a beast, and I must die as a beast."

Historians can debate whether Major Weir truly was the sinful monster he claimed, or whether age and mental derangement had caused him to pour forth his darkest secret fantasies, so long suppressed. In either case, this unnerving scandal left its mark on Scottish culture. Not coincidentally, it was Edinburgh's famous son Robert Louis Stevenson who in 1886 created one of literature's most frightening dual personalities in the Strange Case of Dr. Jekyll and Mr. Hyde.

But long before Stevenson, the Weir affair had inspired another great Scottish writer, who specifically focused on its religious dimensions. James Hogg (1770-1835) was a self-taught writer who deployed an immense knowledge of Scottish history and literature, and whose invented ballads were commonly taken as authentic survivals from ancient times. In 1824, Hogg published The Private Memoirs and Confessions of a Justified Sinner, which has enjoyed very mixed fortunes in terms of its critical reputation. Long virtually forgotten in mainstream English literature, it achieved spectacular renown in Continental Europe, where it was regarded as a Gothic classic as well as a brilliant psychological study of religious hypocrisy.

Through the advocacy of European writers like André Gide, the Anglo-American world rediscovered it in the mid-20th century. As postmodernist approaches became popular, the novel awed critics by its playful attitude to historicity and chronology, its dreamlike surrealism, its pioneering use of multiple narrative perspectives, and by the creation of the ultimate unreliable narrator. Today, the book's classic status is ensured. Quite apart from its well-known virtues, its exploration of the religious roots of violence and fanaticism gives it a contemporary feel, and makes it ripe for yet another popular rediscovery.

Hogg's novel is multiply astonishing, as one of the first literary works to portray violent insanity from the standpoint of a deeply disturbed perpetrator. We see the events initially through the eyes of an editor, who describes the late 17th-century marriage of a worldly Scottish laird and a deeply pious woman. After having two sons, the couple separates, with the strong suggestion that the younger son, Robert Wringhim, is actually the illegitimate son of a hyper-Calvinist local cleric—"one whose righteousness consists in splitting the doctrines of Calvin into thousands of undistinguishable films, and in setting up a system of justifying-grace against all breaches of all laws, moral or divine." When they become adults, Robert stalks his brother and eventually kills him. During a furious investigation, Robert disappears.

So much for the editor's (fairly) sober narrative. At that point, a third of the way through the text, the book moves onto an entirely new and alarming plane with the introduction of Robert's own "Memoirs and Confessions," which reveal the inner mind of a religious fanatic. Hogg had read phenomenally widely in Scottish texts and archives, and knew precisely how devout Kirk members thought and wrote at the relevant time, around 1700. He also drew on such other confessional texts as John Bunyan's Grace Abounding to the Chief of Sinners. Consequently, Hogg's sense of the era's religious discourse is pitch-perfect. At first glance, this is the sort of spiritual diary we could expect to find from a hundred archives in the English-speaking world, dating anywhere from the 17th century though the 19th.

Robert describes his upbringing, portraying himself as a mean-minded priggish hypocrite whose parents delight in his theological sophistries. Indeed, Robert's psychic and moral collapse has its origins in his religious views. Hogg was writing at a time when the Scottish church still retained its established status and much of its popular authority, yet its basic Calvinist tenets were under serious challenge from Enlightenment liberalism. One of the book's principal goals is to satirize and undermine that Calvinism, especially the belief in predestination, and to show how easily it could be carried to bizarre and unhealthy extremes.

Like any good Calvinist, Robert believes in his absolute and unconditional election. His spiritual father, the local parson, convinces him "That I was now a justified person, adopted among the number of God's children—my name written in the Lamb's book of life, and that no by-past transgression, nor any future act of my own, or of other men, could be instrumental in altering the decree." Yet this particular justified sinner is utterly self-righteous and proud, identifying his own interests with God's will. He dismisses all personal rivals or critics as enemies of that divine will, beyond any hope of salvation. They deserve to die.

Although he does not preach Antinomianism, it is the logical outcome of his fevered thinking. As he cannot openly admit to his homicidal instincts, he projects an evil alternate personality. In a moment of revelation, he meets a wonderful stranger whom he first believes to be his guardian angel.

"You think I am your brother," says the newcomer, "or that I am your second self. I am indeed your brother, not according to the flesh, but in my belief of the same truths, and my assurance in the same mode of redemption, than which I hold nothing so great or so glorious on earth." In this new character, Gil-Martin, Robert is overjoyed to find an equally pious soul after his own heart, and with whom he shares intimate dialogues. The two vehemently reject ideas of good works and affirm their common faith in predestination. These discussions lead Robert to experience a parody of a classic conversion experience, an exulting Born-to-Kill-Again moment. Robert's description is densely packed theologically, and lethally logical:

From the moment, I conceived it decreed, not that I should be a minister of the gospel, but a champion of it, to cut off the enemies of the Lord from the face of the earth; and I rejoiced in the commission, finding it more congenial to my nature to be cutting sinners off with the sword than to be haranguing them from the pulpit, striving to produce an effect which God, by his act of absolute predestination, had for ever rendered impracticable.

Not only do Christians waste time trying to convert the ungodly, it is blasphemous even to try:

The more I pondered on these things the more I saw of the folly and inconsistency of ministers in spending their lives striving and remonstrating with sinners in order to induce them to do that which they had it not in their power to do. Seeing that God had from all eternity decided the fate of every individual that was to be born of woman, how vain was it in man to endeavor to save those whom their Maker had, by an unchangeable decree, doomed to destruction. I could not disbelieve the doctrine which the best of men had taught me, and towards which he made the whole of the Scriptures to bear, and yet it made the economy of the Christian world appear to me as an absolute contradiction. How much more wise would it be, thought I, to begin and cut sinners off with the sword! For till that is effected, the saints can never inherit the earth in peace.

Robert's life then takes bizarre and bloody twists. His lengthy dialogues with Gil-Martin give a verbally dazzling portrait of diabolic temptation, to which he increasingly succumbs. (Hogg's older contemporary Goethe was in these same years undertaking his final revisions to Faust, and the two authors would have had much to discuss.) In effect, Robert becomes an assassin for the Devil's cause, targeting those who are self-evidently the enemies of truth and righteousness, as he conceives them.

Robert also finds himself losing periods of time, of which he has no conscious recollection. As he returns from those blackouts, these fugues, he is startled to find that he is accused of many acts of violence and fraud, all of which have clearly occurred, but without his waking knowledge. Someone has been using his body.

As the reader deduces, Gil-Martin, then, is obviously his alter, a splinter personality, though whom he commits his evil deeds, and Hogg leaves heavy clues pointing in that direction. Gil-Martin's interactions with Robert are self-evidently internal and imaginary, existing only in the "Sinner's" own mind.

No sooner have we understood the dual personality theme, though, when Hogg throws a sizable wrench into the novel's mechanism, baffling critics at the time and subsequently. (Hogg liked to play with his readers.) We are perplexed to hear that other characters have also seen Gil-Martin, sometimes together with Robert, so the two are clearly distinct. That would presumably make Gil-Martin the Devil himself or one of his minions. I have no easy explanation for this paradox, unless we are to assume that Robert's alter has assumed material form, and that he has in fact spawned a demon.

Whatever interpretation we ultimately place on Gil-Martin, Hogg is again delving deep into Scottish traditions of deluded confession. Scotland had reported many instances of alleged devil-worship, and perhaps the most quoted example in the entire literature of witchcraft was the wholly spontaneous confession of a young woman named Isabel Gowdie, who presented her revelations only a few years before Major Weir. Something in Scottish religious life was driving a deep and toxic fascination with evil and the Satanic.

At so many levels, this enigmatic novel repays frequent re-reading, but its chillingly plausible anatomy of religious-based violence should make it required reading for anyone interested in contemporary global conflicts. What, for instance, goes through the mind of a self-proclaimed Islamic holy warrior who has sworn allegiance to al-Qaeda or the Islamic State, and who slaughters and terrorizes innocent victims in God's name? Although Sunni Muslims affirm a fundamental belief in Qadar, predestination, the theological underpinnings of such groups are radically different from Robert Wringhim's. Yet on every page of his Confessions, we hear the authentic voice of fanaticism, words that could have come from contemporary Iraq or Syria.

Robert, like those extremists, is the self-appointed vessel of God's wrath, unquestioning in his sense of serving as God's chosen instrument: "There was one thing clear, I was now the Lord's and it behoved me to bestir myself in His service. Oh that I had an host at my command, then would I be as a devouring fire among the workers of iniquity!" And like his modern-day counterparts, he utterly dehumanizes all who fall short of his own faith. They are sheep for the slaughter. Only their death can permit the saints to enjoy the world in peace. "I saw and was convinced that the elect of God would be happier, and purer, were the wicked and unbelievers all cut off from troubling and misleading them."

In imagining Robert Wringhim's mind, James Hogg was recording an imaginary visit to Hell, and reporting what he had seen there. That vision still informs our understanding of the psychology of terror.

Philip Jenkins is Distinguished Professor of History at Baylor University's Institute for Studies of Religion. He is the author most recently of The Great and Holy War: How World War I Became a Religious Crusade (HarperOne).

Copyright © 2015 by the author or Christianity Today/Books & Culture magazine.
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