2 Peter 1:20,21

2 Peter 1:20-21 knowing this first, that no prophecy of scripture is of private interpretation. For no prophecy ever came by the will of man: but men spake from God, being moved by the Holy Spirit.

Expanded Translation

Knowing this first (keeping this foremost in your minds): that no prophecy of Scripture exists (or, is created; or, occurs) of the prophet's own invention (the prophet did not put his personal slants on the message he spoke). For no prophecy ever came (was brought) by the will (purpose, design) of man; rather, men spoke from God, being moved (borne, carried) by the Holy Spirit.

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These verses tell us why the prophecies are worthy of our time and attention; because, when they were given, the Holy Spirit was speaking!

knowing this first, that no prophecy of scripture is of private interpretation

There are at least three basic views concerning the meaning of this phrase.
1. The view of the Roman Catholic Church: Because every part of Scripture was written by men inspired by the Holy Spirit and declared as such by the Church (note the capital C), the Church is also to interpret it. God has promised to guide the Church into all the truth to the end of the world; therefore, we must submit to the judgment of the Church and not depend upon our own fallible and erroneous judgment.[54]

[54] A summary of the footnote in the Duay-Rheims (Catholic) Version of the Bible. The word Church, in Roman Catholic terminology, does not here have reference to the members of that organization, or even to the Priests and Bishops (as a full reading of the quote will show). It refers to The Most Holy Seethe Pope himself!

Thus we are not to trust the meaning we would place on Scripture, for the divine task of rightly dividing the word of truth is in the hands of the Roman Catholic hierarchy, and, finally, in the hands of its head, the pope.

2. That no one can explain prophecy by his own mental power, as it is not a matter of subjective interpretation. To explain it, one needs the same illumination of the Holy Spirit in which it originated. The idea is that Divine assistance is needed. Thus several translators have renderings such as, that no prophecy of Scripture can be understood through one's own (unaided) powers.
That there is a degree of truth to this explanation I would not question, though it seems to present several difficulties. Consider a third possibility:
3. That no prophecy of Scripture exists as a result of one's private or personal views. The reference in the phrase under con sideration is not primarily to the receivers of prophecy, but the writers or speakers of it, the prophets.

The word epilusis (interpretation) is from the verb epiluo, which, as Macknight points out, primarily signified to untie a knot, to unloose a bundle so as to disclose what it contains. The prophets only released the burden God had given them, only untied the bundle and revealed its contents to mankind for all ages to view. So Young's literal translation reads, No prophecy of the Writing doth come of private exposition. The writers of Scripture did not put their own construction on the God-breathed words they wrote.

... the apostle teaches that the truths which the prophets communicated were not originated by themselves; were not their own suggestion or invention; were not their own opinions, but were of higher origin, and were imparted by God; and according to this passage may be explained -... that it is a great principle in regard to the prophets, that what they communicated was not of their own disclosure; that it was not revealed or originated by them,-'Barnes.

for no prophecy ever came by the will of man: but men spake from God, being moved by the Holy Spirit

This verse seems to confirm the rightness of our interpretation of 2 Peter 1:20. No true prophecy ever came because some man willed it or decided to manufacture it. Rather, men spoke as though they were the very mouth of God.

How strongly were the prophets influenced by the Holy Spirit as they spoke? The apostle specifies that they, were moved (phero) by its power. (The same word is rendered came earlier in the verse.) They were borne along, or impelled by the Holy Spirit in their speechnot expressing their own thoughts, but expressing the mind of God in words provided and ministered by Him.[55]

[55] W. E. Vine, An Expository Dictionary of New Testament Words.