A bronze statue of German theologian Martin Luther in Hanover, Germany. His criticism of the Catholic Church in 1517 led to the Protestant Reformation. (AFP/Getty Images)

It was the original viral post.

On Oct. 31, 1517, an obscure German professor of theology named Martin Luther launched an attack on the Roman Catholic Church by nailing his 95 Theses to the door of Wittenberg’s Castle Church — a story that has been repeated for hundreds of years. Luther’s act of rebellion led to the Protestant Reformation, which is being marked by millions of Christians around the world Tuesday on its 500th anniversary.

But did that dramatic moment — Luther defiantly hammering his critique to the church door — really happen?

The story was first told by Philipp Melanchthon, a fellow professor at the University of Wittenberg, a close friend of Luther’s and a leader of the Reformation, after Luther’s death in 1546. And the church door did serve as a public bulletin board of sorts.

But Melanchthon was not in Wittenberg on the day he supposedly witnessed the nailing. He didn’t join the university faculty until 1518. And Luther, a prolific writer who published 30 pamphlets in three years and later translated the Bible into German, never recounted the story.

None An oil-on-wood portrait of Luther, circa 1529. (Ralph Andreas Suchert for The Washington Post)

In 1961, Erwin Iserloh, a Catholic Luther researcher, argued that there was no evidence that Luther actually nailed his 95 Theses to the Castle Church door. Indeed, at the 1617 celebration of the Reformation, Luther was depicted as writing the 95 Theses on the church door with a quill.

Iserloh’s assertion set off a debate among Luther historians that remains unresolved.

A decade ago, Martin Treu, who works for the Luther Memorials Foundation of Saxony-Anhalt, discovered a handwritten note by Luther’s secretary, Georg Rörer, made in a revised copy of the New Testament before Luther’s death. It reads: “On the evening before All Saints’ Day in the year of our Lord 1517, theses about letters of indulgence were nailed to the doors of the Wittenberg churches by Doctor Martin Luther.”

While Rörer was also not an eyewitness, Treu noted, “he was one of Luther’s closest staff.” Treu’s conclusion: 95 Theses may have been nailed to several church doors in Wittenberg, not just at Castle Church.

What’s not in dispute: Luther mailed his attack on the Catholic sale of indulgences to the archbishop of Mainz and Magdeburg, Albert of Brandenburg, on Oct. 31, 1517. The indulgences were meant to assure their buyer that their sins would be forgiven — a form of corruption in Luther’s eyes.

“Disputation on the Power and Efficacy of Indulgences” quickly spread across Europe and reached Pope Leo X sometime in 1518. After a series of disputes, Luther was excommunicated from the Catholic Church on Jan. 3, 1521.

The theologian became a celebrity, and with his celebrity came a following and a new religion: Lutheranism. And the founding symbol of the Protestant Reformation remains the door of Castle Church, now inscribed in bronze with Luther’s 95 Theses.

Read more Retropolis:

Our Lady of Fatima: The Virgin Mary promised three kids a miracle that 70,000 gathered to see

Poland once sent the U.S. a birthday card. With 5 million signatures.

‘The night America burned’: The deadliest — and most overlooked — fire in U.S. history

‘The greatest drug fiends in the world’: An American opioid crisis — in 1908

Mail that baby: A brief history of kids sent through the U.S. Postal Service