The emotional testimony is under way Wednesday in a Philadelphia courtroom.
The Rev. James Brennan is charged with assaulting the man when he was a 14-year-old boy in 1996.
Brennan is on trial with Monsignor William Lynn, who is the first Roman Catholic official in the U.S. charged with endangering children for allegedly keeping predator priests in parish work. Both have pleaded not guilty.
Brennan's lawyers say the accuser has a history of lying and has been in trouble with the law. The accuser testified during cross-examination that he has had psychological problems and grappled with substance abuse as a result of the molestation.