A letter written by a Philadelphia priest four decades ago about teenage brothers on a sadomasochistic outing could help prove that church officials endangered children 30 years later, a prosecutor contended Wednesday.

The Archdiocese of Philadelphia learned about the letter in 1968, but allowed the Rev. John Mulholland to work and minister at parishes across the region until 2002, Assistant District Attorney Mariana Sorensen said.

"It shows that they put blinders on," Sorensen told Common Pleas Court Judge M. Teresa Sarmina.

Prosecutors want to be allowed to tell jurors about decades-old conduct by Mulholland, who no longer is a priest, and dozens of other priests during the March trial for William J. Lynn, the archdiocese's former secretary for clergy. They say the evidence will help them prove that Lynn's actions regarding two other priests, facing trial with him, are part of a long-term pattern or practice by the church of protecting abusive clergy and hiding their conduct from parishioners.

Lynn's church-paid lawyers say most of the allegations - spelled out in secret personnel files and disclosed in a 2005 grand jury report - are unproven, are irrelevant, or could unfairly taint the jury's views.

Lynn faces conspiracy and endangerment charges for recommending parish assignments in the 1990s for the Rev. James J. Brennan and Edward Avery, now also a former priest, despite allegedly knowing or suspecting they would molest children.

In separate incidents, Brennan and Avery are accused of abusing a boy during those assignments. Like Lynn, they have pleaded not guilty.

As secretary for clergy from 1992 to 2004, Lynn recommended priests' assignments and spearheaded investigations into allegations of clergy sex abuse. During the same span, Mulholland repeatedly asked archdiocesan officials to make him a pastor, Sorensen said.

That's when Lynn would have reviewed Mulholland's secret personnel file, including letters he wrote to a young man in the 1960s, she said. The man's mother passed them to church officials after finding them in her 18-year-old son's possessions.

According to Sorensen, one described a naked 15-year-old boy being strung up and flogged in the woods by his brothers, 17 and 18. The prosecutor didn't name the teens or say if she believed Mulholland had witnessed or participated in the outing.

But she said Mulholland routinely took boys on camping trips and wrote in one letter about "a two-week torture treatment" designed to turn himself into "the perfect slave," she said.

"The letters just jump out at you as sick," Sorensen told the judge.

Archdiocesan records say Mulholland served in parishes in Norristown, Philadelphia, Warrington, and Levittown between 1965 and 2002.

Lynn's lawyers countered that Mulholland's case highlighted arguments they had been making for months. The letters came to light a quarter-century before Lynn took his post, they said. Despite no abuse claims against Mulholland, Lynn recommended against making him a pastor and ultimately had a role in his 2002 removal from ministry.

Attorney Thomas Bergstrom said the letter about sadomasochistic teens was "abhorrent" but not proof of a crime. Its only role at trial, he said, would be to inflame a jury.

"The point is, it was fantasy," said Bergstrom. "It never happened."

Sorensen shot back: "This is not fantasy here. This happened. It's real."

She said prosecutors hope to present a trial witness who will testify about Mulholland's sadomasochism, but she did not elaborate.

Sarmina is expected to decide to rule on the prosecutors' request after more arguments on Monday.

The judge said she will also reexamine the issue of Cardinal Anthony J. Bevilacqua's competency at that proceeding. She did not elaborate, and all attorneys are barred from publicly discussing the case under a gag order.

The 88-year-old retired prelate testified in a private pretrial hearing in November. Prosecutors have not said if they plan to introduce that testimony or call Bevilacqua to the stand.

Contact staff writer John P. Martin at 215-854-4774, at jmartin@phillynews.com or at JPMartinInky on Twitter.