It looked like an error.

John Locke, the 17th-century English philosopher known for his antagonism toward Catholics even as he argued for religious tolerance, had little sympathy for those he called “Papists.”

But there it was in Locke’s own script: “Persecution disobliges the best sort amongst the Papists, as well as amongst others.”

In other words: There are good Catholics and society might suffer without them, said Felix Waldmann, a history lecturer at Christ’s College in Cambridge, England.

“I thought to myself, ‘That seems very, very unlikely, indeed,’” said J.C. Walmsley, a scholar in London. “If you know anything about John Locke, it’s that he didn’t tolerate Catholics.”

The surprising declaration was found in a tome unearthed at a surprising location: St. John’s College in Annapolis. There, researchers discovered the 350-year-old theological musings of Locke.

The quest began four years ago, when Walmsley stumbled upon an obscure reference to a listing in a 1928 book collector’s catalogue that named the manuscript: “Reasons for tolerating Papists equally with others.”

The title alone suggested one of two things: Walmsley was looking at a glaring mistake, or he had happened upon something no historian had seen before.

Locke, at a time when England enforced a single religion, argued that the church and the state should be separate entities. He also made the case for religious tolerance, but that idea shunned atheists and never included Catholics.

“He’s very, very consistent in saying Catholics are not eligible” for toleration, Waldmann said about Locke. “That, I think, is why the manuscript on its surface is so unusual.”

As soon as Walmsley had the exact name of the manuscript, he could find the second — and, at that point, only — other reference online. It was on the website of St. John’s College.

Catherine Dixon, library director at St. John’s, helped Walmsley inspect the documents. It started with a digital scan.

“It’s tucked away in a rare book room for safe keeping,” Dixon said. “It’s old, so we wanted to be careful with it.”

After about a year of corresponding with Dixon, Walmsley traveled to Annapolis to see the tattered, yellowed pages for himself.

“I was surprised to find it,” Walmsley said. “It was no doubt at all that this was a Locke manuscript that no one had seen before.”

Dixon said little is known about Henry MacDonald, the New Yorker who donated the manuscript to St. John’s. The University of Oxford owns most of Locke’s work.

Walmsley and Waldmann say they believe Locke wrote the manuscript before he penned his “Essay concerning Toleration.” In the 1667 essay, Locke made the case for religious tolerance as his government persecuted anyone who splintered from the Church of England.

Many of the persecuted fled to the North American colonies — the Puritans to Massachusetts, the Quakers to Pennsylvania and Catholics to Maryland.

Jacqueline Rose, a lecturer at the University of St. Andrews in Scotland, helped Waldmann and Walmsley identify the manuscript as being written by Locke by cross-referencing it with other research about the philosopher.

“It’s really exciting to find this manuscript,” Rose said. “It helps us understand how Locke formulated his ideas.”

Thousands of Locke’s letters, manuscripts, notes and treatises exist online and in library archives, Walmsley said. So how did historians miss this one?

For most of its existence, the manuscript was in private hands. Locke gave it as a gift to a friend before he died.

And while librarians at St. John’s knew the text existed, they probably didn’t understand its significance because historians hadn’t written about it, Walmsley said.

As institutions digitize their records, scholars have an easier time digging up historical nuggets on the Internet.

“A lot of it has to do with pure good fortune on [Walmsley’s] part that St. John’s College digitized it and put it online,” Waldmann said.

While the manuscript has helped historians gain a better understanding of Locke and his theories, Waldmann is hesitant to put Locke on a pedestal.

For one, Locke didn’t view men and women as equal.

“He had no room for the rights possessed by women,” Waldmann said.

Locke also helped write a constitution for the colony in the Carolinas that said white settlers would have absolute authority over enslaved Africans, Waldmann added.

“Locke’s legacy is bound up with a great deal of moral ambiguity,” Waldmann said. “So I think you can selectively admire Locke’s principles, but you should be very careful about holding him up as a figure of uncritical admiration.”