Horatius Bonar emphasizes the singularity of the testimony of Jesus as the central theme of the Bible, highlighting that it is the unified message from one God, delivered through one messenger, and focused on one Savior. He illustrates that the testimony spans from creation to revelation, consistently portraying God's character, humanity's sinfulness, the singular way of salvation, and the hope of resurrection. Bonar warns against idolatry, urging believers to worship God alone and recognize the angelic messenger's role as a servant. The sermon culminates in the understanding that the testimony of Jesus is the spirit of prophecy, binding the entire scripture together in a cohesive narrative. Ultimately, Bonar calls for a recognition of the oneness of God's message throughout history.