More than 100 senior Roman Catholic bishops from around the world will gather in Rome this week for a summit Pope Francis has called to address clerical sexual abuse – the most serious crisis in the church since the Reformation, according to a Catholic historian.

The Vatican has sought to downplay expectations surrounding the four-day meeting, which begins on Thursday. But survivors and advocacy groups say it must deliver clear outcomes if it is to begin to restore the church’s damaged credibility on the issue and avoid being seen as a talking shop.

The removal from the priesthood of the former archbishop and cardinal Theodore McCarrick, one of the church’s most prominent figures, at the weekend sent a strong signal from the Vatican that sexual abuse will no longer be swept under the carpet. Francis on Sunday asked for prayers for the summit, calling abuse “an urgent challenge of our time”.

Although Francis, who will be present throughout the summit and will give a closing speech, has previously warned that expectations must be “deflated”, the senior Vatican figure moderating the conference said last week that the church’s credibility was “strongly at stake”.

Father Federico Lombardi said in Rome: “We must deal with this theme with depth and without fear.”

Conceding there had been “resistance” by some bishops, he added: “If we don’t commit ourselves to fight against these crimes, in society and in the church, then we are not fulfilling our duty.”

Massimo Faggioli, a church historian and professor of theology, has said: “In my opinion, [this is] the most serious crisis in the Catholic church since the Protestant Reformation.” The issue, he added, was being used to “radically delegitimise” Francis’s papacy.

The presidents of 115 bishops’ conferences from around the world have been invited to Rome, along with a dozen heads of eastern Orthodox churches and 10 representatives of women’s religious orders.

Bishops were urged to meet with survivors of sexual abuse in their countries ahead of the conference. Survivors will give testimonies at daily prayer services during the Rome meeting, and vigils and marches organised by advocacy groups are expected to take place outside.

The protection of minors in the church summit comes after a year in which the church was shaken by multiple scandals of sexual abuse and its cover-up by senior members of the church in North and South America, Europe and Australia.

The pope was accused of failing to grasp the seriousness and scale of the problem, and of disregarding widespread rumours of sexual abuse concerning McCarrick. Francis’s conservative enemies seized on the issue to attack his papacy.

But in December, the pope vowed the church would never again cover up sexual abuse, and would spare no effort to bring perpetrators of such “abominations” to justice. He warned abusers to “hand yourself over to human justice, and prepare for divine justice”.

This month, he acknowledged for the first time the sexual abuse of nuns by priests and bishops, saying some had been reduced to “sexual slavery”. In one case, an Indian bishop has been accused of repeatedly raping a nun over a two-year period.

Pope Francis will attend the summit. Photograph: Luca Zennaro/EPA

The abuse scandals that have emerged over the past 20 years have been concentrated in western countries. Very little is known about the scale of abuse in the global south, where the Catholic church has undergone huge growth.

“Many bishops in the global south do not believe that sex abuse of minors is a problem in their countries. They see it as a first world problem,” Thomas Reese, a Jesuit priest and commentator on the church, wrote last month.

“This is in part because many global south bishops have no idea how bad the problem is. In their traditional cultures, victims of abuse are very reluctant to come forward to report the abuse to the church or civil authorities.

“As a result, too many bishops around the world ... deny the problem; they treat it as a sin, not a crime; they don’t listen to the victims; they believe the priest when he says he will never do it again; they keep him in ministry; they cover up.”

The Rome summit, Reese suggested, was primarily aimed at these bishops rather than at the American church.

Despite efforts to play down expectations of the conference, some survivors and church figures hope Francis will explicitly lay down the law to bishops – although they enjoy significant autonomy in their dioceses.

Cardinal Blase Cupich of Chicago, a member of the committee preparing the meeting, said he was confident “we can expect decisive action to follow from the meeting … both in Rome and in dioceses worldwide”.

The summit would “advance a framework of protocols for responding to allegations of clergy sexual abuse”, including identifying “concrete steps required to implement practices that safeguard children and to bring justice to victims in a way that is responsible, accountable and transparent – even in cultures where law enforcement or the government may not be reliable partners,” he told America magazine.

Francis had “acknowledged and apologised for his mistakes, itself an important action, and a model for all bishops” and had made it clear that a bishop “can be legitimately removed from office if he has through negligence committed or through omission facilitated acts that have caused grave harm to others”, Cupich said.

He added: “The very fact that the pope has called this meeting indicates his resolve to make sure there is no doubt about what to do in such matters.”

Although Francis had come in for criticism over his handling of the sexual abuse issue, “it is not a question of what is at stake for the pontificate of Pope Francis, but what is at stake for the church,” Cupich said.