John Gill emphasizes the significance of the voice from heaven heard by Peter, James, and John during the Transfiguration of Christ, which confirmed Jesus' divine glory and authority. This event served as a powerful testimony to the apostles, affirming their faith in Christ's future return in power and glory. Gill explains that the 'holy mount' refers to the mountain where this divine revelation occurred, highlighting the sacredness of the moment. The apostles were not only witnesses to this miraculous event but were also compelled to share the truth of Christ's power and coming based on both their experience and the prophetic word. This sermon encourages believers to trust in the certainty of Christ's return, grounded in both personal experience and Scripture.