Peter the apostle and Paul the teacher of the Gentiles have brought us to know the law of the Lord (the Entrance Antiphon for the Vigil Mass). The apostles Peter and Paul have always been considered, out of tradition, as the principal pillars of the church founded by Christ. This is mainly because of who they were (specifically Peter), as well as the work they did (specifically Paul). Peter had been with Jesus since the inception of the movement and for some reason, conducted himself as the undisputed 'captain' of the team that Jesus had gathered around himself. After the resurrection, Jesus confirmed Peter's leadership role when he handed over to him the reigns of the movement. From that moment onwards, Peter became the 'visible' leader of the group (Jesus remained the leader of the movement, but the group needed someone to be physically in charge). Peter's confirmed new role as the vicar of Christ was that of strengthening the faith of his brothers (cf. Luke 22:32) and shepherding the flock that had been gathered by Christ (cf. John 21:15-17). In the confusion that followed the sudden arrest and execution of Jesus, the eleven turned to Peter for direction and guidance (cf. John 21:3). After the ascension of Jesus, the centrality of Peter’s role became even more visible in his efforts to keep the early church united and committed to the cause for which Jesus had founded her. He carefully strode the two points of view that were emerging in the early church and which were threatening to tear apart the early church (conservatives led by James the elder, and progressives led by Paul). This unifying responsibility shouldered by Peter has been extended to his “vicars,” the popes. 
The second pillar of the Church, Paul of Tarsus, was never part of the twelve who accompanied Jesus in his three or so years of ministry. He might have heard of Jesus while Jesus was still alive, but their paths never crossed. However, Christ did choose and appoint him an apostle, sending him to preach the good news to the gentiles (cf. Acts 9:15). After his conversion, Paul emerged as a very important and valued figure for the advancement of the Jesus’ movement. It was Paul who stood to Peter's face when the latter wanted to bow to pressure from the conservatives concerning the treatment of Christians of gentile background (cf. Galatians 2:11-14). Paul spearheaded the preaching of the Good News into gentile territory, making three missionary journeys in a career that spanned approximately 20 years. He is considered the greatest missionary of all time, the advocate of pagans, and the apostle to the gentiles.
Paul and Peter are rightly considered the principal pillars of the church. However, this is not to say that they were better or more faithful than the “other” pillars who helped them shoulder the weight and responsibility of the nascent Church (apostles and early missionaries). The other apostles were just as dedicated as Peter and Paul were, and most, if not all of them, gave their lives for Christ. Peter and Paul are celebrated as pillars because they were called and they responded. They allowed the Lord to use them as instruments for disseminating the good news even when it meant losing their lives in the process. Peter and Paul never shied away from the responsibility the Lord had given them because they allowed the Word to transform their lives. Their encounter with Christ transformed them into bearers of the Good News of God, a task for which they readily gave their lives. As we celebrate saints Peter and Paul today, let us ask, through their intercession, for the grace to respond generously to the Lord’s call and to remain faithful to it. May our voices, just like theirs, resound to the ends of the earth with the Good News of the Lord Jesus Christ.