Horatius Bonar emphasizes the significance of Christ as the Holy One, the True One, and the possessor of the Key of David in his sermon. He explains that Christ's holiness and truth are foundational to His character, and that He holds the keys to various aspects of life, including the house, castle, city, treasure-house, and banqueting-house of David. Bonar encourages believers to approach Christ for access to these blessings, reminding them that He opens doors that no one can shut and closes doors that no one can open. The sermon serves as a source of comfort and motivation for ministers, churches, and individuals, affirming that Christ's power and grace are always at work. Bonar concludes with a call to respond to Christ's invitation to enter into His presence and blessings.