The Catholic abuse scandal in the United States is approaching a critical moment as the Vatican prepares for a worldwide abuse summit with a prominent American former cardinal under Vatican investigation and another American cardinal under pressure for changing his story about what he knew about the case.

Cardinal Donald Wuerl, whose resignation as archbishop of Washington, D.C., was accepted last year, apologized Tuesday for what he called a "lapse of memory." He acknowledged that he had known about an allegation of abuse against his predecessor, former Cardinal Theodore McCarrick, despite several denials over the past year.

The letter comes as U.S. Catholics' trust in the church is plummeting, according to a new Gallup poll, and as the Vatican is preparing to convene a summit of national church leaders from around the world to discuss the international abuse crisis next month.

"The Church's credibility has been seriously undercut and diminished by these sins and crimes, but even more by the efforts to deny or conceal them," Francis told U.S. bishops in a letter this month laying the groundwork for the gathering, all of which the pope has said he will attend.

Wuerl, 78, who succeeded McCarrick as archbishop of Washington in 2006, remains acting administrator of the archdiocese until a new archbishop is appointed. Meanwhile, Pope Francis ordered McCarrick, 88, who resigned from the College of Cardinals last year, to give up public ministry and to observe "a life of prayer and penance" pending a Vatican trial.

McCarrick has denied the allegations. Both the Vatican and his attorneys have declined to comment.

Wuerl was bishop of Pittsburgh from 1988 to 2006 — a period covered by the blistering state grand jury report in August that found that more than 300 priests had been accused of having abused more than 1,000 children across Pennsylvania since the 1940s.

The report alleged that Roman Catholic Church leaders covered up the crimes and obstructed justice — specifically naming Wuerl as having allegedly concealed abuse by accepting the reassignment of priests who had been accused of abuse to other parishes.

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

After Wuerl was promoted to archbishop of Washington, he ordered a review of McCarrick's records, he said in August, after the Archdiocese of New York announced that it was investigating a "credible and substantiated" allegation of abuse of a male teenager when McCarrick was a priest in New York almost 50 years ago.

Wuerl said that he was "shocked and saddened" and that he had found no claim — "credible or otherwise" — against McCarrick during his time in Washington.

On Saturday, however, Wuerl acknowledged that in 2004, while he was bishop of Pittsburgh, he not only read but passed along to the Vatican's U.S. representative a report detailing an allegation of sexual abuse against a priest by an accuser who was an adult at the time of the alleged abuse. At the end of the report, the man also alleged "inappropriate conduct" toward him by McCarrick during the 1980s, when McCarrick was a top church leader in New Jersey.

After the New Jersey allegation became public, Wuerl denied that he had known of any accusations against McCarrick. Asked in August in an interview with CBS News whether he was "aware of the rumor that McCarrick was having relations with other priests," Wuerl replied: "No. No."

Former Cardinal Theodore McCarrick, the archbishop emeritus of Washington, D.C., at a news conference with senators and religious leaders at the Capitol in Washington in December 2015. Chip Somodevilla / Getty Images file

In a letter to fellow priests on Saturday, Wuerl said that when the allegation against McCarrick became public, he denied any knowledge because the "assertion was in the context of the charges of sexual abuse of minors, which at the time was the focus of discussion and media attention."

"While one may interpret my statement in a different context, the discussion around and adjudication of Archbishop McCarrick's behavior concerned his abuse of minors," while the New Jersey accuser was an adult, he wrote.

The Archdiocese of Washington said in a statement that "the cardinal stands by those statements, which were not intended to be imprecise."

But Wuerl's letter and the archdiocese's statement were widely criticized by Catholic activists and in Catholic media and online commentary, which complained that they were contorting their words to deflect responsibility.

"Well, that's technically true, isn't it?" the prominent Catholic writer Rod Dreher wrote in a commentary in The American Conservative. "'During his time here in Washington.' Of course Donald Wuerl was dissembling. He knew."

The Survivors Network of Those Abused by Priests, or SNAP, said in a statement Monday: "We understand that Cardinal Wuerl has known about sexual abuse allegations against Cardinal Theodore McCarrick since at least 2004. We understand that Cardinal Wuerl reported those allegations to officials in the Vatican but said nothing to local police or prosecutors. We understand that Cardinal Wuerl continues to pretend he had no role in covering up McCarrick’s abuse."

Then, on Tuesday, Wuerl issued a second letter, reiterating that he believed that he had acted "responsibly." He said that when he denied knowledge of allegations against McCarrick last year, "I had forgotten" about the 2004 case.

"Nonetheless, it is important for me to accept personal responsibility and apologize for this lapse of memory," he wrote. "There was never the intention to provide false information."

SNAP responded Wednesday by calling Wuerl's second letter "little more than a lame justification for his actions."

"It is yet another example of a high-ranking church official minimizing his role in cover-ups and excusing his lack of action," the group said.

All of this is playing out as American Catholics are rapidly losing confidence in the church.

In its annual survey of opinion about ethical standards in the clergy, conducted in December, Gallup Inc. found that a record low 31 percent of U.S. Catholics rated the honesty and ethical standards of the clergy as "high" or "very high."

That's behind even journalists (33 percent), and it represents a drop of 18 percentage points from the same survey in 2017. The survey reported a margin of sampling error of plus or minus 4 percentage points.

Noting the coming Vatican summit in February, Gallup declared: "The outcome of that meeting may very well be a deciding factor for many Catholics who are questioning the future of their church.".