Final answer:
Peter is considered the foundation of the church because of the biblical tradition that says Jesus gave him the keys to heaven, leading to the concept of the Petrine Succession where his authority was passed to subsequent bishops and popes in Rome.
Step-by-step explanation:
Peter is considered to be the foundation of the church primarily due to biblical tradition, as it is believed that Jesus Christ gave him the symbolic keys to heaven. This concept, known as the Petrine Succession, is based on the idea that Peter's authority as the first bishop of Rome was handed down to subsequent bishops and popes in the city. Over time, the role of the pope evolved, with the papacy claiming authority over the Church, a claim rooted in historical assertions of power and theological interpretations of Peter's role as outlined in Christian doctrine.