Story highlights The sexual abuse trial for a Philadelphia priest and a parochial teacher has been postponed

The trial had been scheduled to start Tuesday

Both men are accused of raping an altar boy more than a decade ago

The alleged victim will be the key witness

A judge has postponed the trial of a Philadelphia Catholic priest and a parochial school teacher, both accused of raping the same altar boy in separate incidents.

The trial, scheduled to start Tuesday, was postponed due to a family emergency for a defense attorney. A new trial date could be set by the end of the week.

The Rev. Charles Engelhardt, who was a priest at St. Jerome Parish in northeast Philadelphia, and Bernard Shero, a teacher at the parish's school, have been charged with rape, indecent sexual assault and other criminal charges in the alleged assaults that occurred more than a decade ago.

The accuser, dubbed "Billy" in a 2011 Philadelphia grand jury report, will be the key witness against the men, both of whom have pleaded not guilty to the charges.

The former altar boy, now in his 20s, testified earlier this year during the landmark trial of two Philadelphia priests charged with child sexual abuse and conspiracy. It marked the first time a Catholic church leader -- in this case, Monsignor William Lynn -- has been convicted for covering up the crimes of offending priests.

Billy reported to authorities in 2009 that he was sexually abused while he was in Catholic grade school by Engelhardt, Shero and another priest, Edward Avery. A grand jury determined his allegations had merit, and the priests were subsequently charged and arrested. They were among dozens of Philadelphia priests accused of sex abuse in the grand jury report.

According to the grand jury's report, Engelhardt allegedly showed Billy pornographic magazines before having him engage in oral intercourse in the sacristy, the room inside the church where a priest prepares for Mass. The alleged incident happened in 1998 when Billy was 10.

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The following school year, Shero, the boy's sixth-grade teacher at St. Jerome School, is accused of sodomizing the then 11-year-old, according to the grand jury report. Shero allegedly offered the boy a ride home and then stopped at a park. He told the boy they were "going to have fun," took off his clothes, allegedly raped the boy, then made him walk the rest of the way home, the grand jury report stated.

In April, Billy testified at the trial of Lynn, the first high-ranking church figure charged with child endangerment for shuffling predator priests from parish to parish, and the Rev. James Brennan, who was accused of the attempted rape of a 14-year-old altar boy.

Lynn, 61, was found guilty in June of one count of child endangerment and later sentenced to three to six years in prison . The jury was unable to bring a verdict against Brennan, who is scheduled to be retried next year before a new judge.

Avery, who had been scheduled to be on trial with Lynn and Brennan, pleaded guilty after admitting he sexually assaulted Billy when he was 10, and was sentenced to two-and-a-half to five years.

In his April testimony, Billy detailed the alleged abuse at the hands of Avery when he was in the fifth grade. He also testified that the sexual assault led to a life of substance abuse, a suicide attempt and a criminal history including drug possession.

He said he did not tell anyone about the abuse until 2009, after a group therapy session for his drug use. When asked why he didn't speak out earlier, Billy said he was "too scared."

"I thought that I would get into trouble and that no one would believe me," he told jurors in April. "I thought I did something wrong, and, he's a priest."

Priests, particularly those in high-ranking positions, have an exceptional amount of power within the Catholic Church, especially in Philadelphia because of the church's deep roots in the community. With nearly 1.5 million members, the Archdiocese of Philadelphia is one of the largest in the nation. A new archbishop has been installed to help turn around the scandal-plagued archdiocese.