Defense lawyers plan to attack the credibility of the troubled adult accusers when two Roman Catholic priests go on trial in a landmark child sex-abuse case involving the Archdiocese of Philadelphia.

But that strategy took a hit Thursday when co-defendant Edward Avery entered a last-minute guilty plea, confirming one accuser's account of a brutal 1999 sexual assault inside a church sacristy. The victim was then a 10-year-old altar boy, Avery a 57-year-old priest.

Avery's plea leaves Monsignor William Lynn and the Rev. James Brennan on trial on Monday. Brennan, 48, is charged with raping a 14-year-old boy in 1996.

Lynn, 61, handled priest assignments for the archdiocese as secretary for clergy from 1992 to 2004.

He is the first U.S. church official ever charged with endangering children for allegedly failing to oust Avery and Brennan from ministry despite prior child-sex complaints. But he may not be the last.

Philadelphia prosecutors say Lynn helped carry out "an archdiocesan-wide policy ... (that) was criminal in nature." And they've hinted they could charge others when the trial ends.

Civil lawyers believe the trial will help them refile priest-abuse lawsuits that were thrown out in Pennsylvania because of legal time limits, or persuade the state legislature to open a window for filing child sex-abuse claims.

"The evidence that has come out about the conspiracy and the cover-up and the level of officialdom involved in it is going to help us," said lawyer Jay Abramowitch, whose priest-abuse lawsuit involving 18 accusers was thrown out by the state Supreme Court in 2005.

Both Brennan and Lynn have pleaded not guilty. Avery agreed to serve 2-1/2 to five years in prison for involuntary deviate sexual intercourse and conspiracy to endanger a child's welfare. More importantly, he acknowledged that the archdiocese kept him in parish work despite knowing of the earlier complaint.

Lynn remains the focal point of the trial. He faces a long prison term if convicted.

He has argued that he prepared a list of 37 accused priests in 1994, and sent it up the chain to Cardinal Anthony Bevilacqua _ only to have Bevilacqua have it shredded. The cardinal died this year, but his videotaped deposition could be played at trial.

The trial will be followed by concerned Catholics across the country, including some who say their lives were destroyed.

"It gives me hope that it's going to raise public awareness, and it's going to expose the church _ what they knew, when they knew it," said Art Baselice Jr. , a retired Philadelphia homicide detective.

His son, Arthur III, overdosed in 2006, after his civil suit accusing a Philadelphia priest of abuse was thrown out.

The trial will pit priests against fellow priests, and is sure to be painful for Catholics throughout the region.

The Rev. Chris Walsh started the Association of Philadelphia Priests last year, so the 800 priests in the archdiocese can share support and information.

"The priests want the same thing as the lay people," Walsh said Thursday. "We want to know what happened. And, if possible, why it happened.

"The gospel says the truth will set you free. Let's find out what the truth is."