Simplify the following text for a Grade 6 native English-speaking person.
Original: One of the holiest sites of Christianity in the Catholic Tradition , it is traditionally the burial site of its titular Saint Peter , who was one of the twelve apostles of Jesus and , according to Catholic Tradition , also the first Bishop of Antioch , and later first Bishop of Rome , the first Pope .
Simplified: In Catholic tradition , St. Peter 's Basilica is believed to be the burial place of Saint Peter , who was one of the twelve apostles of Jesus . It is believed that Saint Peter was the first Bishop of Rome .