‘Knowing this first, that no prophecy of scripture is of that person's own explanation, for no prophecy ever came by the will of man, but men spoke from God, being moved by the Holy Spirit.'
For they must primarily recognise that no prophecy of Scripture was ever but the personal opinion of the prophet. For no such prophecy ever came as the result of the prophet's decision, but rather men spoke from God as they were moved by the Holy Spirit.
So the power and coming of the Lord Jesus Christ have been evidenced both by the voice of God's majesty on the mount of Transfiguration, as connecting with His honour and glory which was revealed there, ‘we beheld His glory' (John 1:14), and by the word of prophets themselves, moved by the Holy Spirit. ‘Arise, shine, for your light has come, and the glory of the Lord has risen upon you' (Isaiah 60:1). Two voices had spoken. What greater testimony could they have? And this was in total contrast to the false teachers of whom he will now speak.
Others translate as ‘of private interpretation' meaning interpretation by the reader, signifying for example that we must interpret with the help of the Holy Spirit, or that we must agree together concerning the meaning of Scripture, and abjure odd flights of fancy, or that the people must be subject to the Apostolic teaching. All are of course true. But these latter views, although true, do not really tie in with the context of the words.