“What does the Koran say about…?” is perhaps the most common question my students ask me in the Islamic history courses I teach. It’s an understandable question, but they will be disappointed with the answer if they hope it will explain how Islam has been interpreted and practiced for all of history.

In the post-enlightenment West, a society historically influenced by Protestantism’s “back-to-the-Bible” appeal, many of my students have grown up imbibing a public discourse obsessed with a religious or civil tradition’s origins and founding documents—the Hebrew Bible, the New Testament, the Constitution—and by extension often assume that the only book of consequence for Muslims is the Koran. After 9/11, sales of the Koran skyrocketed. More recently, in the wake of the Orlando nightclub shooting, news outlets from Haaretz to Newsweek ran pieces asking “What Does the Koran Say about Being Gay?” And over the past month, as ISIS called for increased attacks during Ramadan, the Koran was again scrutinized as the source of the violence.

While there is no doubt that the Koran is a book of great consequence for Muslims and non-Muslims alike, it’s particularly important now to develop a broad awareness of the many kinds of texts, people, and movements that have helped shape Islam as a living tradition over time.

In the manuscript library at the Suleymaniye Mosque in Istanbul, I recently discovered a manuscript that shows that Islam is always under revision—and that this revision occurred even within a single Islamic book, as its author considered and reconsidered his interpretations over decades of writing.

The manuscript I’d hunted down was a monumental work from 15th-century Cairo that shaped Sunni Islam as we know it today: Ibn Hajar’s Fath al-Bari, “Unlocking the Divine Wisdom,” which explained in rich detail what Sunnis believed were the most authentic reports about Muhammad’s sayings and practices. This massive commentary explained thousands of these reports, or hadith, and on almost every aspect of the human experience: worship, love, war, business, governance, history, law, medicine, and even dental hygiene.

To suggest something of an analogy, Ibn Hajar’s Fath al-Bari is for Sunnis what Thomas Aquinas’s commentaries on the Gospels are for Catholics, or what Rashi’s commentary on the Talmud is for Jews: a monumental intellectual feat that helped reshape the way a religious community viewed its own tradition. After each of these medieval works was written, it became impossible to imagine reading the hadith, the Gospels, or the Talmud in quite the same way. That’s the power of commentary.

In its own day, Fath al-Bari was an instant classic: Sultans from Egypt, Persia, Tunis, Yemen, and India offered generous land revenues and gold dinars in exchange for even a partial copy.

Today, you can find Fath al-Bari for sale in virtually any bookstore where Islamic books are sold, and in virtually every library that acquires books in Arabic. Its authority is so great that it is often invoked by religious figures, from state-appointed muftis to mainstream TV-clerics on Al-Jazeera to ISIS propagandists in their e-magazines. And the book’s popularity immortalized its author; an Egyptian soap opera was recently made about Ibn Hajar, his life’s work, and his world.

Classics have a certain sheen of finality to them. Readers encounter them in their most polished form and often read them as static works, as if they had been written in a single afternoon by their larger-than-life authors. And yet, a work as monumental as Fath al-Bari took at least three decades to complete. Did Ibn Hajar ever reconsider the meaning of a particular hadith, explaining it one way in his 40s and another way in his 70s? Although Ibn Hajar apparently read early drafts of it aloud for an audience of students, it was largely assumed that the only extant copies of Fath al-Bari were these “final” editions. So the world had little way of knowing.

That’s why I had set off to Istanbul: to hunt down an early draft of Fath al-Bari. And after checking the date and copyist of countless electronic scans at the Suleymaniye’s library, I finally found it. There, in the lower portion of the last page, was the manuscript’s date: 1419, 20 years before the final copy of Fath al-Bari was declared complete. The name of the copyist was a student of Ibn Hajar’s, who had hastily scrawled the work down after hearing it aloud from the mouth of the author himself. And the book was marked up with later revisions and additions.

This manuscript version contains significant differences with the versions of Fath al-Bari now circulating in bookstores and libraries. For instance, there is a discussion of a report about the practice of an additional call to prayer in the marketplace on Fridays, one of the finer points of ritual worship that Shiites and Sunnis have long used to debate one another. Was the practice acceptable, as many Sunnis contended, or did one of the caliphs loathed by some Shiites introduce it as a corrupt innovation, inauthentic to Muhammad’s original example?

In this early draft of Fath al-Bari, Ibn Hajar assumed the additional call to prayer was perfectly acceptable, and offered little commentary grappling with the issue of its controversial origins. It’s only in the later revisions made in the margins that we can see that Ibn Hajar was pressed into addressing the issue head-on, scolding his rivals for sowing confusion about who introduced the practice by circulating what he considered unreliable hadith. In retrospect, the point he was making was not so much about the legitimacy of the additional call to prayer as about what kinds of sources Muslims ought to trust and transmit across the generations. Ibn Hajar had not even wanted to include the unreliable reports in his earliest draft, for fear of giving them a platform even if only to debunk them.

Another section of the manuscript shows us that Ibn Hajar continued to expand the meaning of a hadith about the seven kinds of Muslims who will find shelter from God on the Day of Resurrection that each person faces after death. According to the hadith, the seven included a loving friend, a just leader, a charitable person, and so on. In the early draft, it is apparent that for Ibn Hajar, this list of seven was not meant to be taken literally as a limit, as some of his fellow commentators assumed. Rather, he offered another list of seven that would be sheltered by God, beyond the original seven listed in the hadith: a war hero and an honest merchant, among others. In later revisions, Ibn Hajar adds another seven, and yet another! He continually revised his work not only to outdo his rivals, but also to push back against an overly narrow reading of the hadith that would curb the multitude of ways for Muslims to enjoy God’s favor.

In the age of ISIS, it’s tempting to look for the simplest ways to make sense of Islam. But the revelation that Ibn Hajar revised and re-revised his commentary on hadith offers an important reminder: Islam can’t be reduced to a single sacred book, frozen in time. It’s a dynamic and complex tradition that was continually revised and re-revised over many lifetimes, and even within a single lifetime. You might even say that the history of Islam is a history in which Muslims are always reconsidering how the many layers of their textual inheritance square with their present social and political circumstances. The next time my students ask me “What does the Koran say about...?” I hope they will take a cue from this well-worn 15th-century manuscript—marked-up and crossed out over the years—and consider revising their own understanding of this rich tradition.