2 Peter 1:20-21

20 But know this first of all, that no prophecy of Scripture is a matter of one’s own interpretation, 21 for no prophecy was ever made by an act of human will, but men moved by the Holy Spirit spoke from God.

After Peter spoke so highly of the authority of the Scriptures (in v. 19), he wanted here to emphasis the source of that authority. It did not come to us by human intellect, but by God. As men (all the authors of Scripture) were moved by the Holy Spirit they spoke and wrote the words of God (2 Tim. 3:15).