John Nelson Darby expounds on Matthew 16, emphasizing the revelation of Jesus as the Christ and the Son of the living God, contrasting the unbelief of the Pharisees and Sadducees with Peter's divinely revealed faith. He highlights the transition from the Jewish system to the establishment of the assembly, built upon the acknowledgment of Christ's true identity, and warns against the subtle influences of unbelief that can affect even the disciples. Darby illustrates how Peter's confession signifies a pivotal moment in understanding Jesus' mission, which includes suffering and the establishment of God's kingdom on earth. He also discusses the authority given to Peter and the implications of the keys of the kingdom, underscoring the necessity of mortifying the flesh to truly follow Christ. Ultimately, the sermon calls believers to recognize the significance of Christ's identity and the implications for their lives and faith.