– On the final day of Pope Francis’ mission to Ireland, as he issued candid apologies for devastating clerical sex abuse scandals, a former top Vatican diplomat alleged in a letter published Sunday that the pope himself had joined top Vatican officials in covering up the abuses and called for his resignation.

The letter, a bombshell written by Archbishop Carlo Maria Viganò, the former top Vatican diplomat in the United States and a staunch critic of Francis, seemed timed to do more than simply derail the pope’s uphill efforts to win back the Irish faithful, who have turned away from the church in large numbers.

Its unsubstantiated allegations and personal attacks amounted to an extraordinary public declaration of war against Francis’ papacy at perhaps its most vulnerable moment, intended to unseat a pope whose predecessor, Benedict XVI, was the first to resign in nearly 600 years.

Viganò claimed that the Vatican hierarchy was complicit in covering up accusations that Cardinal Theodore McCarrick had sexually abused seminarians and that Francis knew about the abuses by the now-disgraced American prelate years before they became public.

In a news conference on the papal plane back to Rome late Sunday evening, Francis was asked whether there was any truth to the accusations that Viganò had personally informed him in 2013 of McCarrick’s history of abuse, or whether Benedict had in fact sanctioned the American, as the letter claimed. The pope did not deny it, but sidestepped the questions by insisting he would not dignify them with a response.

“I will not say a single word about this,” he said. “I believe the statement speaks for itself. And you have the sufficient journalistic ability to make your conclusions. It’s an act of trust.”

The 7,000-word attack on Francis’ allies in the Vatican, published early Sunday Dublin time by several conservative Catholic outlets antagonistic to Francis, represented a steep escalation in the long-standing, and increasingly caustic, rivalries within the church.

Factions have battled over the direction the church has gone under Francis, with conservatives, especially some American cardinals and bishops, warning that his pastoral and inclusive approach and emphasis on social issues dilute church doctrine and pose a mortal threat to the future of the faith.

The battle is joined

Already on Sunday afternoon, the battle was being joined.

Cardinal Blase Cupich, the archbishop of Chicago who is aligned with Francis and was a target of Viganò’s letter, said in a phone interview that he suspected English speakers had helped Viganò write the letter. He called the pope “a man of integrity.”

“If he makes a mistake, he admits it,” Cupich added. “That’s why I’m convinced this is something that he is going to respond to in an appropriate way.”

Cupich also said the timing of the letter raised questions.

“I’d have to leave it up to him to ask him why he timed the release of this at this moment, particularly if he considered this information so very critical and important,” the cardinal said.

The willingness of the pope and his allies to reach out to gay Catholics has infuriated conservatives, many of whom, like Viganò, blame homosexuals for the sex abuse crisis. The pope has argued that the abuse is a symptom of a culture of privilege and imperviousness among priests who value the church’s traditions over its parishioners.

Last month, Francis accepted the resignation of McCarrick, the first such resignation in living memory, after the New York Times and other news outlets published accounts of the alleged abuse and an internal investigation by the American church deemed credible an accusation that he had sexually abused a minor.

But Viganò alleges that Benedict had already punished McCarrick for his abuse of seminarians and priests. The archbishop writes that Benedict had banned the American cardinal from publicly celebrating mass, from living in a seminary and from traveling to give lectures.

There is no public record of such a sanction, and the allegation has not been confirmed.

Cupich said he was not aware of any restrictions that Benedict put on McCarrick as Viganò asserts.

“How can you have secret restrictions? What does that mean?” Cupich said, adding that it would have been Viganò’s duty as nuncio to inform the American bishops of the restrictions. “Why didn’t he tell us this? Why didn’t he enforce it?”

But Viganò accused Francis of failing to apply the sanctions on McCarrick and instead rehabilitating and empowering him to help choose powerful American bishops, including Cupich.

Viganò despises many of those bishops, who now wield influence and promote Francis’ pastoral approach, and he complained in the letter of being deprived of the voice typically given to a papal nuncio in choosing them. He targeted those bishops and cardinals by name, but saved his most ambitious fire for Francis.

“He knew from at least June 23, 2013, that McCarrick was a serial predator,” Viganò writes of Francis, calling for the pope to step down.

At a 2013 reception in the library of the Apostolic Palace in the Vatican shortly after Francis was elected pope, Viganò was effusive with praise for him, saying his audience was “extremely nice, extremely warm.”

But in the letter, he said he had received an icy reception from Francis. And he said the pope had told him on June 23, 2013: “The bishops in the United States must not be ideologized, they must not be right-wing.” Francis then added, according to Viganò, that they must not be left-wing, “and when I say left-wing, I mean homosexual.”

It was then that Francis asked his opinion of McCarrick, to which Viganò said he had replied: “Holy Father, I don’t know if you know Cardinal McCarrick, but if you ask the Congregation for Bishops there is a dossier this thick about him. He corrupted generations of seminarians and priests, and Pope Benedict ordered him to withdraw to a life of prayer and penance.”

Vatican provocateur

Viganò, who blames gays for the child abuse crisis that has destroyed the church’s standing in many countries, dedicates sections of the letter to outing cardinals who he claims belong to what he characterizes as a pernicious “homosexual current” within the Vatican.

Viganò is no stranger to stirring trouble in the Vatican.

A cultural conservative born into a wealthy family in Varese, Italy, he received the title of archbishop from Pope John Paul II in 1992. He later joined the church’s diplomatic corps, one of the traditional sources of power in the Vatican, which gave him access to much of the information he alleges in the letter. In 2009, he was installed by Pope Benedict XVI as secretary of the governorate of Vatican City State, a position similar to mayor of Vatican City.