Rome (CNN) Pope Francis has acknowledged "with shame and repentance" the Catholic Church's failure to act over sexual abuse by clerics against minors going back decades, writing "we showed no care for the little ones; we abandoned them."

In an unusually blunt letter released by the Vatican on Monday, the Pope wrote, "I acknowledge once more the suffering endured by many minors due to sexual abuse, the abuse of power and the abuse of conscience perpetrated by a significant number of clerics and consecrated persons.

"Looking back to the past, no effort to beg pardon and to seek to repair the harm done will ever be sufficient. Looking ahead to the future, no effort must be spared to create a culture able to prevent such situations from happening, but also to prevent the possibility of their being covered up and perpetuated."

His letter comes in the wake of a Pennsylvania grand jury report that detailed decades of sexual abuses by priests and cover-ups by bishops.

Gruesome accounts of abuse

The report said internal documents from six Catholic dioceses in Pennsylvania -- some held in a secret archive to which only the bishop had a key -- show that more than 300 "predator priests" have been credibly accused of sexually abusing more than 1,000 child victims.

The lengthy catalog of clergy sexual abuses in the report makes for difficult reading. As the grand jurors noted, priests and other Catholic leaders targeted boys, girls and teens.

Some victims were plied with alcohol and groped or molested, the report says. Others were orally, vaginally or anally raped, according to the grand jurors.

Francis' conversations with victims over the years shaped the letter, which points out the need for urgent accountability both for those who committed the abuse and for those who covered it up -- bishops, in many cases, said Vatican spokesman Greg Burke.

"This is about Ireland, this is about the United States and this is about Chile -- but not only. Pope Francis has written to the people of God and that means everyone," Burke said in an audio statement. "It's significant that the Pope calls abuse a crime, not only a sin, and that he asks for forgiveness but he acknowledges that no effort to repair the damage done will ever be sufficient for victims and survivors."

Pope Francis's letter directly referred to the Pennsylvania report, which "detailed the experiences of at least 1,000 survivors, victims of sexual abuse, the abuse of power and of conscience at the hands of priests over a period of approximately 70 years."

"Even though it can be said that most of these cases belong to the past, nonetheless as time goes on we have come to know the pain of many of the victims. We have realized that these wounds never disappear and that they require us forcefully to condemn these atrocities and join forces in uprooting this culture of death; these wounds never go away," the Pope wrote.

"The heart-wrenching pain of these victims, which cries out to heaven, was long ignored, kept quiet or silenced. But their outcry was more powerful than all the measures meant to silence it, or sought even to resolve it by decisions that increased its gravity by falling into complicity."

Looking ahead, the pontiff said the church was working on a "zero tolerance" policy on abuse and coverups. He added, "If, in the past, the response was one of omission, today we want solidarity, in the deepest and most challenging sense, to become our way of forging present and future history."

Bishopaccountability.org, a nonprofit advocacy group that tracks church abuse cases, says lawsuits have forced religious orders and dioceses to pay $3 billion in settlements.

Crucial test for Francis' papacy

The Pope has been under increasing pressure to comment on the nearly 900-page report amid a rapidly escalating sexual abuse crisis that has spread across several continents -- from Australia to Latin America.

While the Vatican finally broke its silence on the report by releasing a damning statement on Thursday, Pope Francis notably made no mention of it during his public sermon in Rome on Sunday.

The crisis presents a crucial test for Francis' papacy, which has stumbled badly at times to address sexual abuse among clergy.

As Cardinal Sean O'Malley of Boston, the Pope's top adviser on sexual abuse, said in a statement last week, "The clock is ticking for all of us in Church leadership."

"Catholics have lost patience with us and civil society has lost confidence in us," he added.

Juan Carlos Cruz, who experienced abuse in Chile and now lives in Philadelphia, said that while the abuse report is heartbreaking, he was encouraged by a change in vocabulary coming from the Pope and out of the Vatican.

"There's new language," said Cruz, who spent a week in May talking to Francis about sex abuse. "They talk about crimes. They talk about a culture of death. They talk about a culture of abuse and cover-up. Before, they were omissions, sins, which is terrible."

Francis' letter, he said, "talks about going to local justice, how bishops don't turn the perpetrators (over) to local justice because they're not obligated to do so, and that is a horrible crime."

At the same time, Cruz is disheartened by what he said was the church's fight against measures to bring clergy to justice, he said. He cited specifically lobbying efforts to derail Pennsylvania state Rep. Mark Rozzi's proposal to suspend the statute of limitations for victims of child sex abuse.

"They have to lobby to help survivors, not to fight them," he said.

This weekend, the Pope will travel to Ireland, which also suffered a sex abuse scandal in 2009. Pope Francis is expected to meet with victims during his visit.

All eyes will also be on whether the Pope publicly addresses the Pennsylvania report.