Over centuries, billions of people have read the Bible. Scholars have spent their lives studying it, while rabbis, ministers and priests have focused on interpreting, teaching and preaching from its pages.

As the sacred text for two of the world’s leading religions, Judaism and Christianity, as well as other faiths, the Bible has also had an unmatched influence on literature—particularly in the Western world. It has been translated into nearly 700 languages, and while exact sales figures are hard to come by, it’s widely considered to be the world’s best-selling book.

But despite the Bible’s undeniable influence, mysteries continue to linger over its origins. Even after nearly 2,000 years of its existence, and centuries of investigation by biblical scholars, we still don’t know with certainty who wrote its various texts, when they were written or under what circumstances.

The Old Testament, or Hebrew Bible, narrates the history of the people of Israel over about a millennium, beginning with God’s creation of the world and humankind, and contains the stories, laws and moral lessons that form the basis of religious life for both Jews and Christians.

For at least 1,000 years, both Jewish and Christian tradition held that a single author wrote the first five books of the Bible—Genesis, Exodus, Leviticus, Numbers and Deuteronomy—which together are known as the Torah (Hebrew for “instruction”) and the Pentateuch (Greek for “five scrolls”). That single author was believed to be Moses, the Hebrew prophet who led the Israelites out of captivity in Egypt and guided them across the Red Sea toward the Promised Land.

Yet nearly from the beginning, readers of the Bible observed that there were things in the so-called Five Books of Moses that Moses himself could not possibly have witnessed: His own death, for example, occurs near the end of Deuteronomy. A volume of the Talmud, the collection of Jewish laws recorded between the 3rd and 5th centuries A.D., dealt with this inconsistency by explaining that Joshua (Moses’ successor as leader of the Israelites) likely wrote the verses about Moses’ death.

“That's one opinion among many,” says Joel Baden, a professor at Yale Divinity School and author of The Composition of the Pentateuch: Renewing the Documentary Hypothesis. “But they're already asking the question—was it possible or not possible for [Moses] to have written them?”

By the time the Enlightenment began in the 17th century, most religious scholars were more seriously questioning the idea of Moses’ authorship, as well as the idea that the Bible could possibly have been the work of any single author. Those first five books were filled with contradictory, repetitive material, and often seemed to tell different versions of the Israelites’ story even within a single section of text.

As Baden explains, the “classic example” of this confusion is the story of Noah and the flood (Genesis 6:9). “You read along and you say, I don’t know how many animals Noah took on the ark with him,” he says. “In this sentence it says two of every animal. In this sentence, he takes two of some animals and 14 of any animals.” Similarly, the text records the length of the flood as 40 days in one place, and 150 days in another.

To explain the Bible’s contradictions, repetitions and general idiosyncrasies, most scholars today agree that the stories and laws it contains were communicated orally, through prose and poetry, over centuries. Starting around the 7th century B.C., different groups, or schools, of authors wrote them down at different times, before they were at some point (probably during the first century B.C.) combined into the single, multi-layered work we know today.

Of the three major blocks of source material that scholars agree comprise the Bible’s first five books, the first was believed to have been written by a group of priests, or priestly authors, whose work scholars designate as “P.” A second block of source material is known as “D”—for Deuteronomist, meaning the author(s) of the vast majority of the book of Deuteronomy. “The two of them are not really related to each other in any significant way,” Baden explains, “except that they're both giving laws and telling a story of Israel's early history.”

According to some scholars, including Baden, the third major block of source material in the Torah can be divided into two different, equally coherent schools, named for the word that each uses for God: Yahweh and Elohim. The stories using the name Elohim are classified as “E,” while the others are called “J” (for Jawhe, the German translation of Yahweh). Other scholars don't agree on two complete sources for the non-priestly material. Instead, says Baden, they see a much more gradual process, in which material from numerous smaller sources was layered together over a longer period of time.

Just as the Old Testament chronicles the story of the Israelites in the millennium or so leading up to the birth of Jesus Christ, the New Testament records Jesus’s life, from his birth and teachings to his death and later resurrection, a narrative that forms the fundamental basis of Christianity. Beginning around A.D. 70, about four decades after Jesus’s crucifixion (according to the Bible), four anonymously written chronicles of his life emerged that would become central documents in the Christian faith. Named for Jesus’s most devoted earthly disciples, or apostles—Matthew, Mark, Luke and John—the four canonical Gospels were traditionally thought to be eyewitness accounts of Jesus’s life, death and resurrection.

But for more than a century, scholars have generally agreed that the Gospels, like many of the books of the New Testament, were not actually written by the people to whom they are attributed. In fact, it seems clear that the stories that form the basis of Christianity were first communicated orally, and passed down from generation to generation, before they were collected and written down.

“Names are attached to the titles of the Gospels (‘the Gospel according to Matthew’),” writes Bible scholar Bart Ehrman in his book Jesus, Interrupted. “But these titles are later additions to the Gospels, provided by editors and scribes to inform readers who the editors thought were the authorities behind the different versions.”

Traditionally, 13 of the 27 books of the New Testament were attributed to Paul the Apostle, who famously converted to Christianity after meeting Jesus on the road to Damascus and wrote a series of letters that helped spread the faith throughout the Mediterranean world. But scholars now agree on the authenticity of only seven of Paul’s epistles: Romans, 1 and 2 Corinthians, Galatians, Philippians, 1 Thessalonians, Philemon. These are believed to have been written between A.D. 50-60, making them the earliest known evidence for Christianity. Authors of the later epistles may have been followers of Paul, who used his name to lend authenticity to the works.

By the 4th century A.D., Christianity had been established as the dominant religion in the Western world, and the New and Old Testaments as its most sacred texts. In the centuries to come, the Bible would only become more central to the lives and faiths of millions of people around the world, despite the mystery surrounding its origins and the ongoing, complex debate over its authorship.