In 2Peter 1:19 Peter claims the prophecies about Christ are more certain and clearer from the standpoint of the Gospel (cf. 1Peter 1:12-13 Romans 15:8). In the same way that the hope of our resurrection is made more certain in Jesus’ resurrection (cf. 1Peter 1:3-4), so the Transfiguration of Christ had made his coming more certain in the minds and hearts of those who were eyewitnesses of it, and for us who believe their word. It is fitting that Peter should be the only New Testament writer to point to the Transfiguration, because it seems he was especially moved by the experience (cf. Mark 9:5-6; Luke 9:33-34). So, what occurred to Jesus on the mount strengthens the prophecies of the coming of the Messiah, causing those who believed them to be expectant.
Peter points to the prophecies of the coming of the Messiah or the Day of the Lord in the Old Testament, saying believers need to take heed (2Peter 1:19; cf. Revelation 1:1, 7, 10). We need to look upon those prophecies as a light in a dark place. Peter is drawing upon Genesis 1:2 where darkness was upon the face of the deep. Had anyone but God been present at that moment, the person there wouldn’t have been able to see anything. It was complete darkness. The same is true of the heart of man as it pertains to the works of God, including the coming of Christ (1Peter 2:9; 2Corinthians 4:6). Unless God reveals himself to us, and what he is doing, it is impossible for anyone to imagine what he is doing or intends to do (Isaiah 64:4; 1Corinthians 2:9)
What is the light that shines, and what is the dark place (2Peter 1:19; cf. John 5:35; Psalm 119:105)? Just as John the Baptist was a light that shone prior to Jesus and predicted his coming (John 5:35; cf. Luke 1:76-79), so too the word of God, that is, the prophecies about the coming Day of the Lord or the coming of the Messiah, shines out as the only light in a dark world to tell us what to expect of God. Those very prophecies about the coming of Christ were being perverted by the false teachers at the time of Peter’s writing (2Peter 3:1-4).
In 2Peter 1:19 Peter points to the day star or the planet Venus that shines out in the morning when it was west of the Sun. It foretold of the coming dawn. Peter’s analogy points to the Gospel he preached concerning the coming Kingdom of God (2Corinthians 4:3-6), which is the coming of the Messiah or the Day of the Lord (cf. Revelation 1:1, 7, 10). Just “…as a light that shines in a dark place until the day dawns, so the day star arises in your hearts.” That is, the Gospel is the day star in the heart of the believer, until the “day dawns” or until the Day of the Lord, which is the coming of the Messiah, the King (cf. 2Samuel 23:3-4).
When he says “no prophecy is a matter of private interpretation” (2Peter 1:20), Peter means that none of the prophets of old spoke out of their own understanding concerning events that they witnessed or concerning what should result from those events (2Peter 1:16, 21). False teachers, on the other hand, who preached the myths, taught out of their own understanding and believed the ancient prophets taught in the same manner. The false teachers invented their doctrines. Nothing was told them by God (cf. Jeremiah 23:16-17; 21-22; 25-27; Ezekiel 13:3). However, if the believer takes heed of the Gospel, nothing will come of false teaching, because the word of God is much greater than the word of men (Jeremiah 23:28-29)
In 2Peter 1:21 Peter used the Greek word phero (G5342) to say that prophecy came (G5342) by “ holy men of God who spoke as they were moved (G5342) by the Holy Spirit.” While this doesn’t tell us exactly how the Holy Spirit did his work, we can understand that the same Greek word is used by Luke to show the wind drove the ship that held Paul and company, while they braved the storm on their way to Rome (Acts 27:15, 17). Jeremiah says that he was overpowered by the word of God within him. He tried to hold it back, because of how he was persecuted for preaching, but he couldn’t endure his own silence (Jeremiah 20:7-9). While holy men of God did use other sources at times (Joshua 10:13; 1Kings 14:19; Luke 1:3), what they used and didn’t use out of those sources was the result of being moved by the Holy Spirit. Men wrote in their own words what the Spirit compelled them to say.