In this photo released on May 4, 2015, by a militant website, which has been verified and is consistent with other AP reporting, Islamic State militants pass by a convoy in Tel Abyad town, northeast Syria. (Militant website via AP)

In a boastful press release after the Paris attacks that left 130 dead earlier in the month, ISIS celebrated its “victory” over the city it called the “lead carrier of the cross.”

“Allah granted victory upon their hands and cast terror into the hearts of the crusaders in their very own homeland,” the group wrote.

The medieval language is a glimpse into the terror group’s little-discussed apocalyptic ideology, one that draws on prophecies written after Muhammad’s death that were used to motivate Muslims to battle by caliphs who lived more than a thousand years ago. While al-Qaida occasionally hinted at these doomsday writings foretelling a grand battle with infidels at the end of the world, ISIS has made them the bedrock of its brand.

These prophecies are so entwined with ISIS’ identity that the group has pasted a line from one of them — “a Caliphate in Accordance with the Prophetic Method” — on the coins it has minted, on the badges soldiers wear and even on a billboard marking the beginning of ISIS territory. In that prophecy, Muhammad said that after a “tyrannical monarchy,” an Islamic caliphate would return. The formation of the caliphate would lead to a grand battle with the West that would bring along the end of the world.

The prophecies ISIS relies on were written dozens and sometimes hundreds of years after Muhammad’s death and are not included in the Quran, as Brookings scholar Will McCants explains in his book “The ISIS Apocalypse.” But the prophecies are widespread and believed by many — one poll in 2012 suggested half of Arabs believed the end of the world was nigh.

In ISIS’ interpretation, the figure of the Mahdi, or “the rightly guided one” will appear to lead final battles against the Western infidels, called Rome, in northern Syria before the end of days. After the day of judgment, only those who supported the Mahdi will be saved. The Mahdi will appear after the establishment of an Islamic caliphate. (Medieval caliphs, attempting to gin up support for their own battles against the Christian crusaders, claimed to be the Mahdi in the past.) In some of the prophecies, Jesus descends from the heavens to assist the Mahdi in his battle against the infidels, who are led by an Antichrist figure.

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ISIS leadership has made key military decisions based upon these prophecies. It fought in the summer of 2014 to take over the small and militarily unimportant village of Dabiq close to Turkey because it is name-checked in one of the prophecies as a place where the final great battles will occur. The infidels are supposed to gather under “80 flags” in this village, before their defeat. (Dabiq is also the name of ISIS’ monthly magazine.)

ISIS’ claim that the apocalypse is nigh is a powerful recruitment tool, not unlike an “Act now! 24 hours only!” sales pitch.

“The belief that the end of the world is coming and you’re going to be fighting on the side of the good guys when the world ends is a powerful motivator,” McCants told Yahoo News. “Young people going to join [ISIS] believe they are participating in a apocalyptic prophecy.”

“The really interesting thing here from the psychological perspective is the sense of urgency,” said John Horgan, a psychologist and terrorism expert at Georgia State University. “They’re sending the message that the forces of evil are about to reach their goals so you need to act now rather than later. You don’t have the luxury of waiting for this to happen.”

CLICK FOR SLIDESHOW ; Medics evacuate an injured person on Boulevard des Filles du Calvaire, close to the Bataclan theater, early on Nov. 14, 2015 in Paris. (Thierry Chesnot/Getty Images)

It’s unknown if the leadership of ISIS really believes in these prophecies or is simply using them to establish legitimacy in the eyes of their supporters. ISIS leader Abu Bakr al-Baghdadi has a Ph.D. in Quranic studies.

McCants tracked down examples of ISIS supporters and members trying to fit current events into the murky and sometimes conflicting timeline of the prophecies. “Thirty states remain to complete the number of eighty flags that will gather in Dabiq and begin the battle,” one jihadi Tweeted, seemingly waiting for more nations to sign on to fight ISIS before the last grand battle could occur and usher in the end of the world.

One can imagine ISIS leaders running into trouble if their apocalyptic vision takes too long and supporters begin asking questions and become impatient.

“Part of why ISIS is trying to goad the West into action is it’s a critical part of fulfilling their prophecies,” Horgan said.

The group has also refrained from explicitly calling its leader al-Baghdadi the Mahdi, which means its followers know they have to wait for him to appear before the world is ending. Sunni and Shiite beliefs about the Mahdi diverge in one key way: Sunni Islam, the branch of the religion ISIS adheres to, posits that the Mahdi, the prophet’s successor, has yet to come. According to the Shiite tradition, the Mahdi came but will remain hidden until he brings justice to the world.

That fighters believe they are fulfilling a grand destiny helps explain why thousands of them have been willing to leave more comfortable lives in nations all around the world to join the dangerous and reviled group. “It’s definitely more cultlike than al-Qaida,” said Karen Greenberg, the director of Fordham’s Center on National Security. “It’s got all the accouterments of a cult.”