From the Treasury of Daily Prayer:
The confession of St. Peter did not arise in the imagination of Peter’s heart but was revealed to him by the Father. The reason this confession is important is seen Jesus’ response: “You are Peter [Greek Petros], and on this rock [Greek petra] I will build my church” (Matthew 16:18). As the people of God in the Old Testament began with the person of Abraham, the rock from which God’s people were hewn (Isaiah 51:1-2), so the people of God in the New Testament would begin with the person of Peter, whose confession is the rock on which Christ would build His Church. But Peter was not alone (the “keys” given to him in Matthew 16:19 were given to all the disciples in Matthew 18:18 and John 20:21-23). As St. Paul tells us, Peter and the other apostles take their place with the prophets as the foundation of the Church, with Christ Himself as the cornerstone (Ephesians 2:20). The confession of Peter, therefore, is the witness of the entire apostolic band and is foundational in the building of Christ’s Church. Thus the Church gives thanks to God for St. Peter and the other apostles who have instructed Christ’s Holy Church in His divine and saving truth.