Isaiah 22: 19-23
The key of the House of David is a symbol of the royal steward’s authority to grant or refuse admittance to the royal presence.  A similar image is used in today’s Gospel when Jesus gives the keys of the kingdom of heaven to Peter.

Romans 11: 33-36
Paul rejoices in the incomprehensible wonder of God’s wisdom and providence.

Matthew 16: 13-20
Peter’s profession of faith in Jesus as the Messiah and the Son of the living God is given to him by divine revelation.