I. Devotion

In November, the Islamic State released an infomercial-like video tracing its origins to bin Laden. It acknowledged Abu Musa'b al Zarqawi, the brutal head of al‑Qaeda in Iraq from roughly 2003 until his killing in 2006, as a more immediate progenitor, followed sequentially by two other guerrilla leaders before Baghdadi, the caliph. Notably unmentioned: bin Laden's successor, Ayman al Zawahiri, the owlish Egyptian eye surgeon who currently heads al‑Qaeda. Zawahiri has not pledged allegiance to Baghdadi, and he is increasingly hated by his fellow jihadists. His isolation is not helped by his lack of charisma; in videos he comes across as squinty and annoyed. But the split between al-Qaeda and the Islamic State has been long in the making, and begins to explain, at least in part, the outsize bloodlust of the latter.

Zawahiri's companion in isolation is a Jordanian cleric named Abu Muhammad al Maqdisi, 55, who has a fair claim to being al-Qaeda's intellectual architect and the most important jihadist unknown to the average American newspaper reader. On most matters of doctrine, Maqdisi and the Islamic State agree. Both are closely identified with the jihadist wing of a branch of Sunnism called Salafism, after the Arabic al salaf al salih, the "pious forefathers." These forefathers are the Prophet himself and his earliest adherents, whom Salafis honor and emulate as the models for all behavior, including warfare, couture, family life, even dentistry.

Maqdisi taught Zarqawi, who went to war in Iraq with the older man's advice in mind. In time, though, Zarqawi surpassed his mentor in fanaticism, and eventually earned his rebuke. At issue was Zarqawi's penchant for bloody spectacle—and, as a matter of doctrine, his hatred of other Muslims, to the point of excommunicating and killing them. In Islam, the practice of takfir, or excommunication, is theologically perilous. "If a man says to his brother, 'You are an infidel,' " the Prophet said, "then one of them is right." If the accuser is wrong, he himself has committed apostasy by making a false accusation. The punishment for apostasy is death. And yet Zarqawi heedlessly expanded the range of behavior that could make Muslims infidels.

Maqdisi wrote to his former pupil that he needed to exercise caution and "not issue sweeping proclamations of takfir" or "proclaim people to be apostates because of their sins." The distinction between apostate and sinner may appear subtle, but it is a key point of contention between al-Qaeda and the Islamic State.

Denying the holiness of the Koran or the prophecies of Muhammad is straightforward apostasy. But Zarqawi and the state he spawned take the position that many other acts can remove a Muslim from Islam. These include, in certain cases, selling alcohol or drugs, wearing Western clothes or shaving one's beard, voting in an election—even for a Muslim candidate—and being lax about calling other people apostates. Being a Shiite, as most Iraqi Arabs are, meets the standard as well, because the Islamic State regards Shiism as innovation, and to innovate on the Koran is to deny its initial perfection. (The Islamic State claims that common Shiite practices, such as worship at the graves of imams and public self-flagellation, have no basis in the Koran or in the example of the Prophet.) That means roughly 200 million Shia are marked for death. So too are the heads of state of every Muslim country, who have elevated man-made law above Sharia by running for office or enforcing laws not made by God.

Following takfiri doctrine, the Islamic State is committed to purifying the world by killing vast numbers of people. The lack of objective reporting from its territory makes the true extent of the slaughter unknowable, but social-media posts from the region suggest that individual executions happen more or less continually, and mass executions every few weeks. Muslim "apostates" are the most common victims. Exempted from automatic execution, it appears, are Christians who do not resist their new government. Baghdadi permits them to live, as long as they pay a special tax, known as the jizya, and acknowledge their subjugation. The Koranic authority for this practice is not in dispute.

Centuries have passed since the wars of religion ceased in Europe, and since men stopped dying in large numbers because of arcane theological disputes. Hence, perhaps, the incredulity and denial with which Westerners have greeted news of the theology and practices of the Islamic State. Many refuse to believe that this group is as devout as it claims to be, or as backward-looking or apocalyptic as its actions and statements suggest.

Their skepticism is comprehensible. In the past, Westerners who accused Muslims of blindly following ancient scriptures came to deserved grief from academics—notably the late Edward Said—who pointed out that calling Muslims "ancient" was usually just another way to denigrate them. Look instead, these scholars urged, to the conditions in which these ideologies arose—the bad governance, the shifting social mores, the humiliation of living in lands valued only for their oil.

Without acknowledgment of these factors, no explanation of the rise of the Islamic State could be complete. But focusing on them to the exclusion of ideology reflects another kind of Western bias: that if religious ideology doesn't matter much in Washington or Berlin, surely it must be equally irrelevant in Raqqa or Mosul. When a masked executioner says Allahu akbar while beheading an apostate, sometimes he's doing so for religious reasons.

Many mainstream Muslim organizations have gone so far as to say the Islamic State is, in fact, un-Islamic. It is, of course, reassuring to know that the vast majority of Muslims have zero interest in replacing Hollywood movies with public executions as evening entertainment. But Muslims who call the Islamic State un-Islamic are typically, as the Princeton scholar Bernard Haykel, the leading expert on the group's theology, told me, "embarrassed and politically correct, with a cotton-candy view of their own religion" that neglects "what their religion has historically and legally required." Many denials of the Islamic State's religious nature, he said, are rooted in an "interfaith-Christian-nonsense tradition."

Every academic I asked about the Islamic State's ideology sent me to Haykel. Of partial Lebanese descent, Haykel grew up in Lebanon and the United States, and when he talks through his Mephistophelian goatee, there is a hint of an unplaceable foreign accent.

According to Haykel, the ranks of the Islamic State are deeply infused with religious vigor. Koranic quotations are ubiquitous. "Even the foot soldiers spout this stuff constantly," Haykel said. "They mug for their cameras and repeat their basic doctrines in formulaic fashion, and they do it all the time." He regards the claim that the Islamic State has distorted the texts of Islam as preposterous, sustainable only through willful ignorance. "People want to absolve Islam," he said. "It's this 'Islam is a religion of peace' mantra. As if there is such a thing as 'Islam'! It's what Muslims do, and how they interpret their texts." Those texts are shared by all Sunni Muslims, not just the Islamic State. "And these guys have just as much legitimacy as anyone else."

All Muslims acknowledge that Muhammad's earliest conquests were not tidy affairs, and that the laws of war passed down in the Koran and in the narrations of the Prophet's rule were calibrated to fit a turbulent and violent time. In Haykel's estimation, the fighters of the Islamic State are authentic throwbacks to early Islam and are faithfully reproducing its norms of war. This behavior includes a number of practices that modern Muslims tend to prefer not to acknowledge as integral to their sacred texts. "Slavery, crucifixion, and beheadings are not something that freakish [jihadists] are cherry-picking from the medieval tradition," Haykel said. Islamic State fighters "are smack in the middle of the medieval tradition and are bringing it wholesale into the present day."

The Koran specifies crucifixion as one of the only punishments permitted for enemies of Islam. The tax on Christians finds clear endorsement in the Surah Al-Tawba, the Koran's ninth chapter, which instructs Muslims to fight Christians and Jews "until they pay the jizya with willing submission, and feel themselves subdued." The Prophet, whom all Muslims consider exemplary, imposed these rules and owned slaves.

Leaders of the Islamic State have taken emulation of Muhammad as strict duty, and have revived traditions that have been dormant for hundreds of years. "What's striking about them is not just the literalism, but also the seriousness with which they read these texts," Haykel said. "There is an assiduous, obsessive seriousness that Muslims don't normally have."

Before the rise of the Islamic State, no group in the past few centuries had attempted more-radical fidelity to the Prophetic model than the Wahhabis of 18th‑century Arabia. They conquered most of what is now Saudi Arabia, and their strict practices survive in a diluted version of Sharia there. Haykel sees an important distinction between the groups, though: "The Wahhabis were not wanton in their violence." They were surrounded by Muslims, and they conquered lands that were already Islamic; this stayed their hand. "ISIS, by contrast, is really reliving the early period." Early Muslims were surrounded by non-Muslims, and the Islamic State, because of its takfiri tendencies, considers itself to be in the same situation.

If al-Qaeda wanted to revive slavery, it never said so. And why would it? Silence on slavery probably reflected strategic thinking, with public sympathies in mind: when the Islamic State began enslaving people, even some of its supporters balked. Nonetheless, the caliphate has continued to embrace slavery and crucifixion without apology. "We will conquer your Rome, break your crosses, and enslave your women," Adnani, the spokesman, promised in one of his periodic valentines to the West. "If we do not reach that time, then our children and grandchildren will reach it, and they will sell your sons as slaves at the slave market."

In October, Dabiq, the magazine of the Islamic State, published "The Revival of Slavery Before the Hour," an article that took up the question of whether Yazidis (the members of an ancient Kurdish sect that borrows elements of Islam, and had come under attack from Islamic State forces in northern Iraq) are lapsed Muslims, and therefore marked for death, or merely pagans and therefore fair game for enslavement. A study group of Islamic State scholars had convened, on government orders, to resolve this issue. If they are pagans, the article's anonymous author wrote,

II. Territory

Tens of thousands of foreign Muslims are thought to have immigrated to the Islamic State. Recruits hail from France, the United Kingdom, Belgium, Germany, Holland, Australia, Indonesia, the United States, and many other places. Many have come to fight, and many intend to die.

Peter R. Neumann, a professor at King's College London, told me that online voices have been essential to spreading propaganda and ensuring that newcomers know what to believe. Online recruitment has also widened the demographics of the jihadist community, by allowing conservative Muslim women—physically isolated in their homes—to reach out to recruiters, radicalize, and arrange passage to Syria. Through its appeals to both genders, the Islamic State hopes to build a complete society.

In November, I traveled to Australia to meet Musa Cerantonio, a 30-year-old man whom Neumann and other researchers had identified as one of the two most important "new spiritual authorities" guiding foreigners to join the Islamic State. For three years he was a televangelist on Iqraa TV in Cairo, but he left after the station objected to his frequent calls to establish a caliphate. Now he preaches on Facebook and Twitter.

Cerantonio—a big, friendly man with a bookish demeanor—told me he blanches at beheading videos. He hates seeing the violence, even though supporters of the Islamic State are required to endorse it. (He speaks out, controversially among jihadists, against suicide bombing, on the grounds that God forbids suicide; he differs from the Islamic State on a few other points as well.) He has the kind of unkempt facial hair one sees on certain overgrown fans of The Lord of the Rings, and his obsession with Islamic apocalypticism felt familiar. He seemed to be living out a drama that looks, from an outsider's perspective, like a medieval fantasy novel, only with real blood.

Last June, Cerantonio and his wife tried to emigrate—he wouldn't say to where ("It's illegal to go to Syria," he said cagily)—but they were caught en route, in the Philippines, and he was deported back to Australia for overstaying his visa. Australia has criminalized attempts to join or travel to the Islamic State, and has confiscated Cerantonio's passport. He is stuck in Melbourne, where he is well known to the local constabulary. If Cerantonio were caught facilitating the movement of individuals to the Islamic State, he would be imprisoned. So far, though, he is free—a technically unaffiliated ideologue who nonetheless speaks with what other jihadists have taken to be a reliable voice on matters of the Islamic State's doctrine.

We met for lunch in Footscray, a dense, multicultural Melbourne suburb that's home to Lonely Planet, the travel-guide publisher. Cerantonio grew up there in a half-Irish, half-Calabrian family. On a typical street one can find African restaurants, Vietnamese shops, and young Arabs walking around in the Salafi uniform of scraggly beard, long shirt, and trousers ending halfway down the calves.

Cerantonio explained the joy he felt when Baghdadi was declared the caliph on June 29—and the sudden, magnetic attraction that Mesopotamia began to exert on him and his friends. "I was in a hotel [in the Philippines], and I saw the declaration on television," he told me. "And I was just amazed, and I'm like, Why am I stuck here in this bloody room?"

The last caliphate was the Ottoman empire, which reached its peak in the 16th century and then experienced a long decline, until the founder of the Republic of Turkey, Mustafa Kemal Atatürk, euthanized it in 1924. But Cerantonio, like many supporters of the Islamic State, doesn't acknowledge that caliphate as legitimate, because it didn't fully enforce Islamic law, which requires stonings and slavery and amputations, and because its caliphs were not descended from the tribe of the Prophet, the Quraysh.

Baghdadi spoke at length of the importance of the caliphate in his Mosul sermon. He said that to revive the institution of the caliphate—which had not functioned except in name for about 1,000 years—was a communal obligation. He and his loyalists had "hastened to declare the caliphate and place an imam" at its head, he said. "This is a duty upon the Muslims—a duty that has been lost for centuries … The Muslims sin by losing it, and they must always seek to establish it." Like bin Laden before him, Baghdadi spoke floridly, with frequent scriptural allusion and command of classical rhetoric. Unlike bin Laden, and unlike those false caliphs of the Ottoman empire, he is Qurayshi.

The caliphate, Cerantonio told me, is not just a political entity but also a vehicle for salvation. Islamic State propaganda regularly reports the pledges of baya'a (allegiance) rolling in from jihadist groups across the Muslim world. Cerantonio quoted a Prophetic saying, that to die without pledging allegiance is to die jahil (ignorant) and therefore die a "death of disbelief." Consider how Muslims (or, for that matter, Christians) imagine God deals with the souls of people who die without learning about the one true religion. They are neither obviously saved nor definitively condemned. Similarly, Cerantonio said, the Muslim who acknowledges one omnipotent god and prays, but who dies without pledging himself to a valid caliph and incurring the obligations of that oath, has failed to live a fully Islamic life. I pointed out that this means the vast majority of Muslims in history, and all who passed away between 1924 and 2014, died a death of disbelief. Cerantonio nodded gravely. "I would go so far as to say that Islam has been reestablished" by the caliphate.

I asked him about his own baya'a, and he quickly corrected me: "I didn't say that I'd pledged allegiance." Under Australian law, he reminded me, giving baya'a to the Islamic State was illegal. "But I agree that [Baghdadi] fulfills the requirements," he continued. "I'm just going to wink at you, and you take that to mean whatever you want."

To be the caliph, one must meet conditions outlined in Sunni law—being a Muslim adult man of Quraysh descent; exhibiting moral probity and physical and mental integrity; and having 'amr, or authority. This last criterion, Cerantonio said, is the hardest to fulfill, and requires that the caliph have territory in which he can enforce Islamic law. Baghdadi's Islamic State achieved that long before June 29, Cerantonio said, and as soon as it did, a Western convert within the group's ranks—Cerantonio described him as "something of a leader"—began murmuring about the religious obligation to declare a caliphate. He and others spoke quietly to those in power and told them that further delay would be sinful.

Cerantonio said a faction arose that was prepared to make war on Baghdadi's group if it delayed any further. They prepared a letter to various powerful members of ISIS, airing their displeasure at the failure to appoint a caliph, but were pacified by Adnani, the spokesman, who let them in on a secret—that a caliphate had already been declared, long before the public announcement. They had their legitimate caliph, and at that point there was only one option. "If he's legitimate," Cerantonio said, "you must give him the baya'a."

After Baghdadi's July sermon, a stream of jihadists began flowing daily into Syria with renewed motivation. Jürgen Todenhöfer, a German author and former politician who visited the Islamic State in December, reported the arrival of 100 fighters at one Turkish-border recruitment station in just two days. His report, among others, suggests a still-steady inflow of foreigners, ready to give up everything at home for a shot at paradise in the worst place on Earth.

In London, a week before my meal with Cerantonio, I met with three ex-members of a banned Islamist group called Al Muhajiroun (The Emigrants): Anjem Choudary, Abu Baraa, and Abdul Muhid. They all expressed desire to emigrate to the Islamic State, as many of their colleagues already had, but the authorities had confiscated their passports. Like Cerantonio, they regarded the caliphate as the only righteous government on Earth, though none would confess having pledged allegiance. Their principal goal in meeting me was to explain what the Islamic State stands for, and how its policies reflect God's law.