The voice of the man - youthful, tall, gaunt, and blond - cracked several times as he described McCormick's reaction when his advances were rebuffed.
"He got up and told me to get out," the witness said. "I put my clothes on and walked home by myself."
The Inquirer does not publish the names of sex-assault victims.
Questioned by Assistant District Attorney Kristen Kemp, the man said that after the incident, McCormick accosted him several times, trying to humiliate and shame him.
"He would tell me that homosexuality and masturbation were sins," the witness testified. "He made me feel horrible. He said I was going to hell."
McCormick, 57, has pleaded not guilty and last week rejected a plea deal.
In his opening statement, defense attorney William J. Brennan Jr. told the jury that it should not trust the memory of the accuser, who has had years of emotional problems and drug and alcohol abuse.
The alleged victim said he became more troubled after the incident, attempting suicide, and using alcohol and then illegal drugs. He said he has been sober since 2011.
In her opening statement, Kemp said McCormick targeted the boy after his mother asked for his help because she thought her son was gay, and she and her husband had separated.
"They were thrilled, they were ecstatic," Kemp said of the parents after the priest took the boy under his wing.
The accuser said the incident left him ashamed of what happened and his own sexuality. He said he later told a favorite cousin that he had been molested and, later, fellow students on a church retreat.
But he did not name McCormick until December 2011, nine months after Catholic Church officials announced that McCormick and 25 other priests were being suspended for possible inappropriate contact with minors. McCormick remains a priest, but is not allowed to publicly perform his office or be involved in parish activities.
When he was arrested, he was pastor of Sacred Heart Parish in Bridgeport, Montgomery County. He now lives with his parents in Pottstown.
After telling his parents and his grandfather, a retired police detective, the alleged victim went to the District Attorney's Office. McCormick was arrested in July 2012.
Questioned by Brennan's cocounsel, Richard J. Fuschino, the accuser denied plans to sue the Archdiocese of Philadelphia.
"I have a full-time job," replied the witness, a business manager for a New York company. "I don't need money. I have a very successful career."
The alleged victim said he was moved to contact authorities after a nightmare about McCormick molesting his young nephew.
"I don't want this to happen to any other little boy," he added.