Does Donald Trump have a solid grasp of the ISIS apocalypse? It’s a question no one had reason to ask until last week, when Pope Francis unleashed his inner takfiri by declaring anyone who preferred building walls rather than bridges “not Christian.” (He did not mention Trump by name, but one thing you lose when you assume the Pontificate is the ability to subtweet in peace.) Trump retorted within minutes, in a speech on Kiawah Island in South Carolina. The Vatican, he said, is “ISIS’s ultimate trophy,” and the pope should beg God for Trump’s protection. “The Vatican [will be] attacked by ISIS,” he prophesied, in a statement whose grammar suggested it was translated by computers into Cantonese and back before publication. “I can promise you that the Pope would have only wished and prayed that Donald Trump would have been President because this would not have happened.”

The apocalyptic narratives of Islam differ from those of Christianity, and even many Muslims aren’t familiar with the end-times prophecies of their faith. So Trump’s apparent misunderstanding or partial ignorance of a key element of ISIS ideology is understandable. Indeed, for a man like Trump, any awareness of the prominence of “Rome” in Islamic scripture is a bit surprising—like discovering that Taylor Swift knows how to tie a perfect trucker’s hitch, or that one’s cat can operate the dishwasher. But his claim that Rome is “ISIS’s ultimate trophy” deserves correction, just in case he thinks it’s true rather than merely a fun way to troll the pontiff.

The Arabic word Rum is cognate with the English word “Rome” and is usually translated as such, although on modern maps the city of Rome will generally be written as Ruma. The word appears in the Koran and in the sayings of the Prophet Muhammad, typically rendered in English as “Rome.” These instances include prophecies of battles against “Roman” armies in Asia Minor and (most famously) Dabiq, the Syrian agricultural hamlet that inspired the name of ISIS’s magazine:

The Last Hour would not come until the Romans would land at Al-A’maq [a valley northwest of the Syrian city of Aleppo] or in Dabiq. An army consisting of the best soldiers of all the Earth at that time will come from Medina to counteract them. [Sahih Muslim 2897]

The Islamic State constantly harps on this series of battles. Only once they have occurred will the Antichrist appear, and only after his appearance can the world be rocked by tribulation and saved by the descended Jesus of Nazareth. For the true believers of ISIS, these stories provide an imaginative backdrop to hyperdramatize what is otherwise just another awful Middle Eastern war.

One of the most-quoted statements by Abu Muhammad al-Adnani, the spokesman of the Islamic State, cited another hadith:

We will conquer your Rome, break your crosses, and enslave your women, by the permission of Allah, the Exalted. This is His promise to us; He is glorified and He does not fail in His promise. 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.

On the authority of ‘Abdullah Ibn ‘Amr Ibn al-‘As, who said, “We were with [Muhammad] writing down what he was saying, when he was asked, ‘Which of the two cities will be conquered first? Constantinople or Rome?’ So Muhammad said, ‘The city of Heraclius will be the first to be conquered.’” Meaning the city of Constantinople. It was reported by al-Hakim in “Al-Mustadrak” according to the conditions of the two sheikhs (Bukhari and Muslim) and declared authentic by Imam adh-Dhahabi.

Islamic State propaganda at times encourages a straightforward, Trumpian reading of the word “Rome.” Last year, the Islamic State’s Tripoli province explicitly menaced the city of Rome. In a video depicting the simultaneous beheading of 21 Coptic Christians, the camera lingers over the image of Christian blood mixing with the tides of the Mediterranean. Soon after, ISIS supporters on social media began using the hashtag #We_Are_Coming_O_Rome (until Italians hijacked the tag and clogged it with travel advice and assorted ridicule). A film made by ISIS’s Nineveh propaganda bureau in December depicted an Islamic State-flagged tank crossing a desert to capture the Colosseum. In the distance is the Vittoriano.

There is a Scalia-like originalism in some forms of scriptural interpretation: Find out what “Rome” meant at the time of God’s revelation of the Koran to Muhammad, and you’ll know its One True Meaning. (The analogy to U.S. legal traditions is imperfect, but it’ll do.) Islam has its Breyers and Ginsburgs as well, and they ask the obvious questions: Do these prophecies mentioning “Rome” really refer to the “Rome” known to an illiterate seventh-century Arab merchant? Or are the prophecies “living” documents that require constant reinterpretation?

If we hew closely to what Muhammad meant by “Rome,” we couldn’t easily construe the word to mean the modern, geopolitically trivial Italian capital. Indeed, Rum probably referred not to the city of Rome at all, but to the Eastern Roman (or Byzantine) Empire, centered in Constantinople. According to this interpretation, the Rome of La Dolce Vita and spaghetti carbonara has nothing to do with Rum, and the bluster from Trump and ISIS propagandists rests on a simple misreading.

But that leaves believers with a quandary of a whole other sort, since the Byzantine Empire fell in 1453 and is therefore no longer around to fight. It’s as if Muhammad predicted that the Brooklyn Dodgers would play the New York Yankees in the 2020 World Series. At some point, Muslims concerned with these prophecies have to figure out how a defunct franchise (the Brooklyn Dodgers ceased to exist when they moved to Los Angeles in 1958) will field a team.

And here’s where things get metaphysical. If Muhammad was not wrong, how might he be right? One method of reckoning with this obstacle—the non-existence of “Rome” in the seventh-century sense—is to ask which latter-day entity most resembles Rome. Apocalyptic theorists today have suggested that Rome might be NATO, “the West,” America, Russia, or something else.

One of the Islamic State-linked ideologues I interviewed for my March 2015 story about the group has taken unusual interest in this question. Last year, Musa Cerantonio, a young Australian convert, published a pamphlet in which he purported to answer the question “Who is Rum?” definitively. He considers whether Rome can be identified simply by finding the entities that inhabit the same geography as the Byzantines, or that use their language, or come from their tribe, or use their same form of government. But he rejects all these possibilities, on the grounds that even during the early decades of Islam, the territory, language, and forms of government of the Byzantines varied a great deal, and the name Rum never changed. The essence of Rum must be deeper.

Cerantonio notes that the Ottomans, who defeated the Byzantines, accepted the mantle of “Rome” explicitly: The sultan called himself the “Caesar of Rome” and believed he was establishing a third Roman Empire. And when the Ottoman line ended in 1924—leaving the world without a caliph—their successors were the secular republicans of Mustafa Kemal Ataturk. Those successors still reign in Istanbul and Ankara, and the Islamic State considers them apostates, even though the most recent of them, Recep Tayyip Erdogan, is generally counted as an Islamist by those who aren’t members of ISIS.

For Cerantonio, that settles it: The Rum of the end-times hadith is not the Rome of Pope Francis but the Rome of the Republic of Turkey. Almost two weeks ago, when press reports suggested that the Turkish military had crossed the border near the Syrian city of Aleppo, into the town of Azaz, a social-media account supportive of Cerantonio’s Rum thesis tweeted the news out excitedly: Azaz is smack in the middle of Dabiq/A’maq territory, exactly where the armies of Rome and Islam are meant to meet—right on schedule.

Cerantonio’s word on this topic is hardly final, even for Islamic State devotees: Only God knows what “Rome” is. But his wrestling with these questions gives a sense of the tenor of the Islamic State’s discussion of prophecy, which is not about if but when, where, and who. And that discussion remains active, with participants marshaling evidence from scripture, history, linguistics, and other fields to support their own theory of the end of times. Trump’s interpretation of “Rome”—identifying it with the modern city, and describing it as a key ISIS target—reads the Islamic State’s favorite prophecies even more narrowly than many of its own followers do.

Taking the Islamic State’s religious claims seriously does not mean reading them simplistically—and indeed, reading these prophecies mindlessly can be more dangerous than not reading them at all. Many have noted the Dabiq prophecy and suggested that the United States should grant ISIS’s wish and meet its fighters there in battle, to wipe them out and demonstrate that their religious interpretation is a false one. But anyone who has spent time around the hyper-religious will know that their interpretations can be rigid at times and flexible at others. The failure of one prophecy will as likely as not lead to its reinterpretation, rather than the repudiation of all the other ones. That some elements in the Islamic State are thrilled about an attack on the Italian capital, and others are thrilled about an attack by Turkey in Syria—all on the basis of the same scriptures—shows that this is an enemy whose ideology can’t be easily summarized or used against it. But if your main goal is to use a gross simplification of that ideology to fashion a single anti-papal zinger, it will certainly suffice.