The group met to pray ahead of a worldwide meeting on the clergy sex abuse crisis next month.
Pope Francis said the sex scandals have gravely affected the credibility of the Catholic Church.
"The Church's credibility has been seriously undercut and diminished by these sins and crimes, but even more by the efforts made to deny or conceal them," the pope wrote in his letter.
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He wrote that efforts to create new committees or "improving flow charts, as if we were in charge of a department of human resources" to handle the crisis are insufficient. He said those efforts reduce the mission of the church and "risk reducing everything to an organizational problem."
"God's faithful people and the Church's mission continue to suffer greatly as a result of abuses of power and conscience and sexual abuse, and the poor way that they were handled, as well as the pain of seeing an episcopate lacking in unity and concentrated more on pointing fingers than on seeking paths of reconciliation," the pope said.
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Pope Francis wrote that he told cardinals in the fall he wanted to come to the meeting, but "despite my best efforts, I will not be able, for logistical reasons, to be physically present with you."
Just in: Pope Francis writes forceful letter to bishops on retreat saying he wanted to come to Mundelein with them but “despite my best efforts, I will not be able, for logistical reasons, to be physically present with you.” @Pontifex @ABC7Chicago pic.twitter.com/KA8NGfpy28— Ross Weidner (@RossWeidner) January 3, 2019
US Catholic bishops pray over clergy sexual abuse scandal
The retreat began a day after The Associated Press reported that the Vatican blocked U.S. bishops from taking measures last year to address the scandal because U.S. church leaders didn't discuss the legally problematic proposals with the Holy See enough beforehand.
The rebuke from Rome was contained in a letter from a Vatican official before the U.S. Conference of Catholic Bishops met in November. The move stunned abuse survivors and some other Catholics demanding actions.
The retreat also is a prelude to a summit of the world's bishops at the Vatican next month to forge a comprehensive response to the crisis that has lashed the church.
The meetings follow two blistering reports during 2018 from state attorneys general - in Illinois and Pennsylvania - alleging negligence by state church leaders.
Here's a look at the retreat.
WHAT'S ON THE AGENDA?
This is about prayer, not policy-making, organizers say.
According to the Archdiocese of Chicago spokeswoman Anne Maselli, bishops gathering at the Mundelein Seminary will be praying, fasting and participating in spiritual lectures. And they will be alone: No staff members, other priests or members of the public or media are invited. Cardinal Daniel DiNardo, president of the U.S. Conference of Catholic Bishops, said in a news release that they are convening "to pray on the intense matters before us."
The Catholic seminary at the University of St. Mary of the Lake in Mundelein, 40 miles (64 kilometers) north of Chicago, is the largest of its kind in the U.S. and home to roughly 200 seminarians from about 40 dioceses across the country and globe. According to its website, the lakefront campus blends "Colonial Revival and the architecture of Renaissance Rome, joining the Roman traditions of Catholicism with American cultural traditions."
Tied to the gate at the seminary's entrance were white ribbons with signs beneath them that read: "These ribbons represent prayers for our church." One ribbon bore the words: "Pope Francis & priests for mercy & healing"
A sign also advised that the campus was closed for a private retreat and that trespassing was prohibited.
WHO ARE THE MAIN PLAYERS?
Pope Francis has dispatched Fr. Raniero Cantalamessa, the official papal preacher, to lead the retreat. And it's no accident that it's being held in Chicago, long considered a center of American Catholicism. The hosting Chicago archbishop, Cardinal Blase Cupich, was Francis' first major U.S. appointment and was picked by the pope to help organize the Vatican summit.
Cupich, who is considered a moderate, was the lead signatory on a recent letter to bishops around the world warning that a failure to deal with abuse now will jeopardize the church's mission globally. It also urged summit attendees to meet with clergy sexual abuse victims "to learn firsthand the suffering they have endured." Cupich issued a statement expressing regret for "our failures to address the scourge of clerical sexual abuse," after Illinois Attorney General Lisa Madigan's report in December alleging that the church had failed to disclose the names of at least 500 clergy members in the state accused of sexually abusing children.
A Pennsylvania grand jury report early last year alleged that hundreds of priests abused at least 1,000 children over seven decades in that state.
WHAT CAN IT REALLY ACCOMPLISH?
Potentially a good deal, according to Notre Dame researcher and teacher Timothy O'Malley. He says one of Francis' biggest concerns has been that the bishops experience a spiritual renewal - and "a spirit of penance" - along with regulations governing their behavior.
"When the bishops meet in public to discuss these procedures, there is a danger that it's less an act of contrition and more an occasion for scoring political points," he said. "Part of the corruption is based in a certain clerical culture where bishops pursued self-interest, whether their own or their diocese's, at the expense of listening to lay victims. This retreat ... is a first step toward a renewal of the (church leadership) as a whole in the United States."
O'Malley added that it only works if they recognize that their office isn't about accruing power but becoming "a shepherd," or "someone who is willing to engage in self-sacrifice for the sake of the Church."
HOW DO ABUSE VICTIMS FEEL ABOUT IT?
Many are dismayed that it has taken so long for the church leadership to meet and act after so many years. Two advocacy groups, the Survivors Network of those Abused by Priests (SNAP) and Ending Clergy Abuse, held a public demonstration Wednesday in Chicago.
SNAP says it wants Cardinal Cupich removed from his prominent role in planning the papal summit, and that Cardinal DiNardo should not lead the U.S. delegation to the Rome summit.
Zach Hiner, SNAP's executive director, said he's glad leaders are gathering "to find ways to address this crisis, but a week spent in silent prayer is not the response the public is looking for."
"If church officials truly want to do what is best, then they should be spending this week discussing how they can best comply with independent investigations by law enforcement, or how they can compel law enforcement officials in their state to act if no such investigation has begun yet," Hiner said in an email. "At this point, regardless of what bishops decide to do in the first week of January, we are placing our hopes for reform in the hands of secular, not church officials."
Hiner was joined at the demonstration by Peter Isely, a clergy abuse victim and founding member of both groups. He said most survivors want church leaders "to succeed at this."
"We want to be able to love the Catholic Church and love justice at the same time," Isely said. "If we have to make a decision .... we're going to love justice first until the Catholic Church joins us in our love for justice."
The Associated Press contributed to this report.