Verses 19, 20 and 21 emphasize the reliability of God's word and its prophetic message. Many nonbelievers and critics of the bible are under the false presumption that it was written by men, without any assistance from the Holy Spirit.
As believers, we know that the Holy Spirit is God and that the bible is indeed inspired and written by God who used men to deliver the message to the world. What an awesome gift we have been given! Jesus in form of a man to demonstrates perfection and righteousness and the Holy Spirit to continue to deliver the message of eternal life through Him via faith in the gift of grace and mercy.
19: We also have the prophetic message as something completely reliable, and you will do well to pay attention to it, as to a light shining in a dark place, until the day dawns and the morning star rises in your hearts.
20: Above all, you must understand that no prophecy of Scripture came about by the prophet's own interpretation of things.
21: For prophecy never had its origin in human will, but prophets, though human, spoke from Goad as they were carried along by the Holy Spirit.