Interpret means to 'to explain the meaning of (something). : to understand (something) in a specified way' (Merriam-webster dictionary)
Revelation means: 'a : an act of revealing or communicating divine truth. b : something that is revealed by God to humans.' (Merriam-webster dictionary)
I'm not saying there's anything wrong with interpretation of the Scriptures. I believe it is only wrong when the interpretation is devoid of Divine understanding - which comes via revelation. And that's one way you can tell the difference. If your understanding has come from God, it is revelation. But if your understanding has come from yourself, your own reasoning, then it is your own interpretation. And what does the bible say about such interpretation?
'...knowing this first of all, that no prophecy of Scripture comes from someone's own interpretation. For no prophecy was ever produced by the will of man, but men spoke from God as they were carried along by the Holy Spirit.' (1 Peter 1:20-21)
Revelation, I believe, is necessary, to understand the spiritual meanings of Scripture. Would Philip have been able to correctly interpret the Scriptures, to the Ethopian, without having had revelation from God?
Revelation is certainly needed before one can correctly interpret. But what many carnal men are doing - through such practices as exegesis and hermeneutics - is trying to interpret the Word of God, without God. And in doing so, they arrive at many dreadfully wrong conclusions.
One of the main differences between revelation and personal interpretation is that revelation is objective, and comes from God; while personal interpretation is subjective, and comes from one's own mind.
If you are reading the bible and arrive at a conclusion through your own reasoning; then that is surely due to personal interpretation. On the other hand, have you arrived at a conclusion which you could not have reached by yourself? Then that is, I believe, because of revelation.
What Peter was saying in that passage was that the Scriptures were even more reliable then Peter's and his fellow Apostle's eyewitness of the Lord Himself; then their having walked with Him.
He is saying that the Scriptures are more sure then our sense of sight, touch, and all the rest because the Scriptures did not originate in man; are not men's ideas.
He is saying that this is so because God set apart the men who wrote the Scripture for the express purpose of having them do just that - of revealing His will to them through writings He moved them to write.
How words are used, when and where they are, etc., reveal the meaning intended by the person using those words.
There, in those passages, the word "private interpretation" refers to "own origin."
Note what, who [Peter] Peter is talking about when he uses that phrase, where he does, as this gives us the sense of how he is using it.
I am doing all this through the passages - not through revelation other than what the passages, by how they used words, are giving men their intended sense, or meaning.
Peter first starts off aware that he will soon be gone. The issue for him is a continuing witness that all can rely on - the written Word.
Study Church history and what you find is the notions of men. What you find is that Peter's words here were ignored as to what is to be our only source of wisdom as to the things of God - the Scripture.
Paul said the same thing in 2 Corinthians 4:
13. We having the same spirit of faith, according as it is written, I believed, and therefore have I spoken; we also believe, and therefore speak;
14. Knowing that he which raised up the Lord Jesus shall raise up us also by Jesus, and shall present us with you.
Now, note Peter's concern...
2 Peter 1:
13. Yea, I think it meet, as long as I am in this tabernacle, to stir you up by putting you in remembrance;
14. Knowing that shortly I must put off this my tabernacle, even as our Lord Jesus Christ hath shewed me.
15. Moreover I will endeavour that ye may be able after my decease to have these things always in remembrance.
By what means and on what sure ground, Peter?
16. For we have not followed cunningly devised fables, when we made known unto you the power and coming of our Lord Jesus Christ, but were eyewitnesses of his majesty.
He is saying they did not make these things up. He then expands on that thought he is trying to get across:
17. For he received from God the Father honour and glory, when there came such a voice to him from the excellent glory, This is my beloved Son, in whom I am well pleased.
18. And this voice which came from heaven we heard, when we were with him in the holy mount.
19. We have also a more sure word of prophecy; whereunto ye do well that ye take heed, as unto a light that shineth in a dark place, until the day dawn, and the day star arise in your hearts:
He then goes into why it is a more sure word of prophecy - that they might be able to continue to remember these things by after Peter is gone:
20. Knowing this first, that no prophecy of the scripture is of any private interpretation.
21. For the prophecy came not in old time by the will of man: but holy men of God spake as they were moved by the Holy Ghost.
A lost person could understand what I have just laid out. But, he will conclude it foolishness unto him. That "that was just one of those men saying that back there."
The lost person can understand it but where the rubber meets the road, so to speak, is "in believing."
The lost man will not know it to be true because that requires believing that it is. For, what evidence, according to Hebrews 8:1, does a "knowing" faith require?