Private Interpretation of Scripture

Looking at this passage:

2 Peter 1:20-21 Knowing this first, that no prophecy of the scripture is of any private interpretation. 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.

Protestant Understandings of this Passage

Like most other subjects, you can find Protestant traditions with all sorts of views on this passage. The following two examples are directly contradictory.

Hanover Baptist, for instance, sees it as relating to teaching authority in the early church.

Cults are built upon erroneous interpretations of the Bible. Many disciples have been duped to follow charisma and showmanship.

In the days of the apostles, God assigned a special gift to some of the early saints in order to assist in keeping fidelity and vigilance to God’s truth. It was called the gift of discernment (1 Corinthians 12:10). 

Proclaim and Defend sees the passage in terms of the origin of Scripture, rather than interpretation of Scripture.

Private origination would be a better way of saying it and it is a remark about where the scriptures originated, not their meaning or how they are used.

Neither of those pages do much to place the passage in its context.

2 Peter 1:18 And this voice which came from heaven we heard, when we were with him in the holy mount. 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: Knowing this first, that no prophecy of the scripture is of any private interpretation. 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.

The bold text above in the passage has nothing to do with written Scripture - at least in this context. It's about the words that were spoken by the Apostles. They Apostles were eyewitnesses to the glory of Jesus seen on Mount Tabor where they saw Jesus transfigured before their own eyes.

The passage does highlight the verbal aspect of the message of the Apostles. They testified to what they saw. The message would later be written down and in fact this Epistle itself is an example of that writing.

OSB Commentary on 2 Peter 1:19-21

From the Orthodox Study Bible notes:

The testimony of the Apostles both confirms prophetic word (v 19) concerning the second coming and shows us how to interpret prophecy. Just as Scripture (v 20) was not written by the mere volition of men but by the inspiration of the Holy Spirit (v 21), so Scripture is to be interpreted by Holy men guided by the Holy Spirit. Heretics (ch 2) and unstable Christians (3:16) interpret incorrectly. The Apostles (the we of v 19) are guided by the Holy Spirit, trusting in the promise of true interpretation (John 16:13). The Church, founded by the Apostles, likewise receives the Holy Spirit, 

"For the Apostles, like a rich man in a bank, deposited her with [the Church] most copiously everything which pertains to the truth. And everyone who wishes, draws from her the drink of life. For she is the entrance to life, while all the rest are thieves and robbers. That is why it is surely necessary to avoid them, while cherishing with the utmost diligence the things pertaining to the church, and to lay hold of the traditions of truth (Iren).