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ROME — Pope Francis has penned the preface to a new book by a survivor of clerical sexual abuse, and asked forgiveness for what he called an "absolute monstrosity" in the Church's history.

"It is difficult for a victim of pedophilia to speak out about what they have endured and to describe the trauma that still persists many years later," Pope Francis writes in "Father, I Forgive You" by Daniel Pittet, a Swiss man who was repeatedly raped by a Capuchin friar as a child.

"I am happy that others today can read his testimony and discover to what extent evil can enter a servant of the church. How can a priest, at the service of Christ and the Church, cause so much pain?” the pope writes.

The book will be published in Italy on Thursday but La Republica newspaper ran the pope's preface in full on Monday.

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While Francis has spoken out against clerical sexual abuse and asked for forgiveness to victims and their families before, his latest word on the subject is uniquely personal.

The deaths of victims by suicide "weigh heavy on my heart, on my conscience and on the Church as a whole," Francis writes.

"To their families I offer my feeling of love and pain, and I ask, humbly, forgiveness," he adds. "It is an absolute monstrosity, a horrendous sin, radically opposite to what Christ teaches.”

Since his election in 2013, Pope Francis has promised a “zero tolerance” policy against clerical sexual abuse. He has since established a pontifical commission for the protection of minors, which sets guidelines on how to prevent abuse.

Pope Francis celebrates a mass in Rome on Jan. 21. Tony Gentile / Reuters

He also established a Vatican tribunal to judge bishops accused of covering up sexual abuse or failing to prevent it, although it was later scrapped after it was criticized as window dressing by victims. He has also met abuse victims on several occasions.

In 2015 he ordered the trial and defrocking of Jozef Wesolowski, a Polish archbishop accused of abusing children in the Dominican Republic. Wesolowski died of natural causes while under house arrest in the Vatican.

In the past abuse victims associations have greeted the pope’s words with a degree of suspicion, claiming they were insufficient to curb the crime and bring down the veil of secrecy that allowed priests to perpetuate their abuse unpunished for decades.

A spokesman for SNAP, the world's biggest support group for survivors of clerical abuse, had not seen the book so declined to comment on the pope's comments.

Still, Francis is credited with having held high-level clerics accountable — a point he emphasized in the preface to the book: "It is our duty to prove our severity with priests who betray their mission, and with those, including bishops and cardinals, who protect them, as it has happened in the past."