Williams said that McCormick assaulted the boy inside the rectory at St. John Cantius Church in Bridesburg.
To keep him quiet, the priest reportedly told the boy, "Masturbation is a sin, homosexuality is a sin, premarital sex is a sin, and lying is a sin."
An investigation by the D.A.'s office and the police Special Victims Unit led to an arrest warrant for McCormick, 56, who turned himself in Thursday.
He's been charged with involuntary deviate sexual intercourse, statutory sexual assault, child endangerment and related counts.
McCormick was arraigned Friday, his bail was set at $150,000 and his preliminary hearing is scheduled for Aug. 17, the D.A.'s office said.
His arrest comes in the same week that Monsignor William Lynn became the highest-ranking Catholic Church official in the U.S. to be sent to prison for taking part in covering up clergy abuse of children. A Philadelphia judge on Tuesday sentenced Lynn to three to six years in state prison for his conviction on one count of child endangerment.
McCormick was one of 21 priests placed on administrative leave by the Archdiocese of Philadelphia in March 2011 in response to concerns noted in a February 2011 grand-jury report.
"The cases of those placed on leave at that time ranged from allegations of sexual abuse to boundary issues with minors. Since that time, Father McCormick has not been permitted to exercise his public ministry, administer any of the Sacraments, or present himself publicly as a priest," the Archdiocese of Philadelphia said in a statement.
McCormick's alleged victim was a student, parishioner and altar boy at St. John Cantius at the time of the assault, said Capt. John Darby, of the Special Victims Unit.
McCormick, ordained in 1982, last served at Sacred Heart Parish in Swedesburg, Montgomery County, from 2004 until his removal in March 2011.
He met his alleged victim while serving at St. John's from 1986 to 2000. He then moved to St. Bede the Venerable Parish, in Holland, from 2000 to 2004.
There is reason to believe that McCormick victimized other youths, said Williams.
He urged anyone with information about crimes committed by McCormick or any other priest to call the Special Victims Unit at 215-685-3251 or 215-685-3265 or the D.A.'s Family Violence and Sexual Assault Unit at 215-686-8080.
Contact Mensah M. Dean at 215-568-8278 or firstname.lastname@example.org or on Twitter at @mensahdean.