High Quality Content by WIKIPEDIA articles! The Cathedral Basilica of Saints Peter and Paul, head church of the Roman Catholic Archdiocese of Philadelphia, is located at 18th Street the Benjamin Franklin Parkway on the east side of Logan Square in Philadelphia, Pennsylvania. It is the largest Catholic church in Pennsylvania and listed on the U.S. National Register of Historic Places as Cathedral of Saints Peter and Paul. In 1979, the Cathedral was visited by Pope John Paul II, where he went on to celebrate Mass.