ROME — At 9:30 a.m. last Wednesday, Archbishop Carlo Maria Viganò showed up at the Rome apartment of a conservative Vatican reporter with a simple clerical collar, a Rocky Mountains baseball cap and an explosive story to tell.

Archbishop Viganò, the former chief Vatican diplomat in the United States, spent the morning working shoulder to shoulder with the reporter at his dining room table on a 7,000-word letter that called for the resignation of Pope Francis, accusing him of covering up sexual abuse and giving comfort to a “homosexual current” in the Vatican.

The journalist, Marco Tosatti, said he had smoothed out the narrative. The enraged archbishop brought no evidence, he said, but he did supply the flair, condemning the homosexual networks inside the church that act “with the power of octopus tentacles” to “strangle innocent victims and priestly vocations.”

“The poetry is all his,” Mr. Tosatti said.

When the letter was finished, Archbishop Viganò took his leave, turning off his cellphone. Keeping his destination a secret because he was “worried for his own security,” Mr. Tosatti said, the archbishop then simply “disappeared.”