Philadelphia (CNN) -- A tearful former altar boy testified Wednesday that one of two Philadelphia priests on trial in a clergy abuse case molested him during an overnight visit at the priest's apartment.
The witness, now in his 30s, cried on the stand as he described the incident with the Rev. James Brennan, who is accused of raping the witness in 1996, when he was 14. The man is a former Marine, who was discharged because of mental health issues.
"I was a little boy. I didn't know what to do," the man testified through sobs.
The witness testified before Common Pleas Judge Teresa Sarmina that Brennan, who he described as a close family friend he considered to be like an uncle, viewed online sex chat rooms with him and suggested they masturbate together before insisting the witness sleep with the priest in the same bed.
"I was scared. I was afraid that if I said no he'd kill me or something," he said, crying.
While in bed together and wearing only T-shirts and boxers, the witness testified that Brennan hugged him from behind and pressed his genitals against his buttocks.
He said he prayed he would fall asleep and recalled wetting himself, but does not remember anything after that.
Brennan, 48, is on trial with Monsignor William Lynn in the first case in which a Roman Catholic archdiocese official is accused of covering up evidence of suspected sexual abuse of children. Both men pleaded not guilty.
Lynn, 61, who served as the secretary for clergy under former Philadelphia Archbishop Anthony Bevilacqua, is charged with endangering the welfare of a child in connection with the alleged assaults.
A number of alleged victims of clergy abuse have testified since the trial began March 26, but Brennan's accuser is the first whose claim falls within the statute of limitations. He is part of a 2011 Philadelphia grand jury report.
The grand jury report accused the archdiocese of failing to investigate claims of sexual abuse of children by priests.
The report led to the Philadelphia District Attorney's Office criminally charging four Philadelphia priests and a parochial school teacher with raping and assaulting boys in their care, while Lynn was accused of allowing the abusive priests to have access to children.
On Wednesday, the witness also testified to another incident in 1999, when Brennan exposed himself to the witness while in a shed on the church grounds.
He described a life of substance abuse, suicide attempts and a criminal history including robbery and fraud that he testified came as a result of the alleged sexual assault by Brennan.
"When his collar came down, he was Jim," he said. "Prior to the molestation, everything was cool."
The witness' testimony ended abruptly when he asked for a break after nearly two hours of intense cross-examination by Brennan's attorney, William Brennan, no relation.
In a motion filed Tuesday and argued in court by Brennan's other attorney, Richard Fuschino, the witness is described as "a serial confabulator" with a history of lying to law enforcement, plus having a sordid past of drug abuse.
But prosecutors challenged that claim.
"This is just a smear campaign against a rape victim," argued Mark Cipolletti, assistant district attorney.
Brennan was removed from active ministry in 2006, after the witness first came forward with his allegations. The witness will continue his testimony on Thursday.
Defrocked priest Edward Avery was slated to go on trial with Lynn and Brennan, but he pleaded guilty last week to involuntary deviate sexual intercourse and conspiracy to endanger the welfare of a child, according to court documents.
Avery was sentenced to 2½ to five years in prison. He reported for his sentence on Monday.