For four years, Karen L. King, a Harvard historian of Christianity, has defended the so-called “Gospel of Jesus’s Wife” against scholars who argued it was a forgery. But Thursday, for the first time, King said the papyrus—which she introduced to the world in 2012—is a probable fake.

She reached this conclusion, she said, after reading The Atlantic’s investigation into the papyrus’s origins, which appears in the magazine’s July/August issue and was posted to its website Wednesday night.

“It tips the balance towards forgery,” she said.

Critics had argued for years that errors in Coptic grammar, similarities with the Gospel of Thomas, and other problems pointed to forgery. But King had placed her faith in the opinions of expert papyrologists, along with a series of carbon-dating and other scientific tests, at MIT, Harvard, and Columbia, that had turned up no signs of modern tampering or forgery.

When I called her in March while reporting my Atlantic story, she said she was not interested in commenting on—or even hearing about—my findings before publication. Thursday afternoon, however, she called me to say the story was “fascinating” and “very helpful.” ...