ROME — The Vatican cardinal who co-authored a bombshell book with Emeritus Pope Benedict XVI reaffirming priestly celibacy on Tuesday strongly denied he manipulated the retired pope into publishing.

Guinean Cardinal Robert Sarah, who heads the Vatican’s liturgy office, spoke out after news reports quoting “sources close to Benedict” claimed the retired pope never saw or approved the finished product.

Sarah reproduced letters from Benedict making clear the 92-year-old pope had written the text and approved of publishing it as a book. “These defamations are of exceptional gravity,” Sarah tweeted.

The controversy underscores the conservative-progressive battle lines that have exploded in the Catholic Church following Benedict’s 2013 decision to retire, and his successor Pope Francis’s more reform-minded papacy.

Benedict’s involvement in the book, From the Depths of Our Hearts, had the appearance of being an attempt to interfere with Francis’s ministry. Francis has said he will publish a document in the coming weeks that is expected to touch on whether married men could be ordained priests in the Amazon, to deal with a priest shortage there.

Benedict’s reaffirmation of the “necessity” of priestly celibacy in the book gave the impression that the former pope was trying to influence the thinking of the current one.

His intervention was all the more surprising, given he had vowed to live “hidden from the world” when he retired in 2013, specifically to avoid any suggestion that he still wielded papal authority.

The book includes an introduction and a conclusion, said to be written jointly by Benedict and Sarah, as well as a chapter apiece.

Catholic Twitter accounts, amplifying the rift between right and left, buzzed with the implications of Benedict’s participation.

Francis’s supporters claimed Benedict had been manipulated by members of his right-wing entourage into writing something that amounted to a frontal attack on Francis. Some claimed it was evidence of elder abuse, given Benedict’s 92 years and increasing frailty.

Conservatives, many of whom long for Benedict’s orthodoxy, claimed it was no such thing and noted that Francis too has reaffirmed the “gift” of priestly celibacy.

The Vatican tried to tamp down the whole furor by insisting the book was a mere “contribution” to the discussion about priestly celibacy written by two bishops in “filial obedience” to Francis.

Sarah denied there was any manipulation on his part and said Benedict was very much a part of the process.

He tweeted three 2018 letters from Benedict making clear the retired pope had provided him the text. As he wrote in the book, Benedict said he had begun writing “some reflections on the priesthood” before Sarah even proposed the book in September, but had put the project aside because of his waning strength.

Benedict wrote in a Sept. 20 letter that he had finished his work after Sarah proposed a text on the priesthood and celibacy. The third letter from Benedict, dated Nov. 25, reads “From my side, the text can be published in the form you have foreseen.”

In a statement Tuesday, Sarah chronicled all his interactions with Benedict and said he had approved the final text, including the joint introduction, conclusion and cover. He said he had told Benedict the text would be published after Christmas.

“I sincerely forgive all those who defamed me or wanted to put me in opposition to Pope Francis,” Sarah said. “My attachment to Benedict XVI remains intact and my filial obedience to Pope Francis absolute.”

The “sources close to Benedict” who suggested that the retired pope hadn’t co-authored the book have not been identified. But Benedict’s entourage is small, and is headed by his longtime secretary, Archbishop Georg Gaenswein, who also is head of Francis’s papal household.

Benedict’s letters to Sarah make clear the retired pope had written a text prior to the Amazon synod, in which a majority of bishops backed ordaining married men as priests. After the synod concluded, Francis announced he would write his own reflections on its outcome.

The U.S. publisher of the book, Ignatius Press, which has been Benedict’s English-language publisher since before he became pope, has defended the book and Benedict’s participation in it.

Jesuit Father Joseph Fessio, Ignatius’s founder and editor, said he had read the text in French and English, and said only Benedict could have written it.

“Is it really believable that Sarah would perpetrate a fraud?” Fessio asked in an email. “Or alter something from Benedict? Or claim that Benedict didn’t collaborate and agree to the intro and epilogue?”

Ignatius in November published another book by Sarah, The Day is Now Far Spent, in which the cardinal — a hero to liturgical purists and conservatives and a quiet critic of Francis — again defended priestly celibacy.

In the book, Sarah criticized the proposal to ordain married men in the Amazon, saying it actually showed disdain for the indigenous population and suggested God couldn’t find real priests among them willing to make the sacrifice of celibacy.

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