This week, another trial about allegations of sexual abuse in Philadelphia Catholic schools begins. It has national implications.

The Rev. Charles Engelhardt of the Oblates of St. Francis de Sales and former Philadelphia Archdiocese teacher Bernard Shero are accused of abusing a student at St. Jerome's parish in Northeast Philadelphia in the 1990s.

Cases such as this bring out evidence that has been kept hidden by the Catholic Church, says David Clohessy, executive director of the Survivors Network of those Abused by Priests.

"These kinds of trials are very, very rare," Clohessy said. "So they provide a really unique opportunity to see firsthand what the evidence is about how much Catholic officials knew about abuse and yet tolerated, minimized, ignored it and, in some cases, enabled it."

Clohessy says few allegations of abuse by church employees come to either civil or criminal trial.

"They are settled out of court, if it's a civil case," he said. "There's a plea bargain if there is a criminal case and the higher-ups in the Church get away scot-free."

The Philadelphia archdiocese became an exception to that statement in 2012. A city jury convicted Monsignor William Lynn of child endangerment, saying he failed to prevent priests suspected of abuse from being moved to new parishes.

The current case stems from the same grand jury report as the Lynn charges. Lynn is appealing his three- to six-year prison sentence.

Jury selection for the trial of Engelhardt and Shero is expected to take a week or more.