John Calvin Commentary 2 Peter 1:20

"knowing this first, that no prophecy of scripture is of private interpretation." — 2 Peter 1:20 (ASV)

Knowing this first. Here Peter begins to show how our minds are to be prepared, if we really wish to make progress in scriptural knowledge. There may at the same time be two interpretations given, if you read ἐπηλύσεως as some do, which means occurrence, impulse; or, as I have rendered it, interpretation, ἐπιλύσεως. But almost all give this meaning: that we ought not to rush rashly when we read Scripture, trusting in our own understanding. They think that a confirmation of this follows, because the Spirit, who spoke by the prophets, is the only true interpreter of himself.

This explanation contains a true, godly, and useful doctrine: that prophecies are read profitably only when we renounce the mind and feelings of the flesh and submit to the teaching of the Spirit, but that it is an impious profanation of it when we arrogantly rely on our own insight, believing that sufficient to enable us to understand it, though the mysteries contain things hidden to our flesh, and sublime treasures of life far surpassing our capacities. And this is what we have said: that the light which shines in it comes only to the humble.

But the Papists are doubly foolish when they conclude from this passage that no interpretation by a private individual ought to be considered authoritative. For they pervert what Peter says to claim for their own councils the chief right of interpreting Scripture; but in this they indeed act childishly. For Peter calls interpretation private, not that of every individual in order to prohibit each one from interpreting; but he shows that whatever people contribute of their own is profane. Even if, then, the whole world were unanimous, and if the minds of all people were united, still what would proceed from them would be private, or their own. For this term is here contrasted with divine revelation, so that the faithful, inwardly illuminated by the Holy Spirit, acknowledge nothing but what God says in his word.

However, another meaning seems simpler to me: that Peter is saying Scripture did not come from humanity, or through human suggestions. For you will never come well prepared to read it unless you bring reverence, obedience, and teachableness; but a proper reverence exists only when we are convinced that God speaks to us, and not mortal humans. Therefore, Peter especially urges us to believe the prophecies as the undoubted oracles of God, because they have not originated from people's own private suggestions.

What immediately follows serves the same purpose:

But holy men of God spake as they were moved by the Holy Ghost. They did not of themselves, or according to their own will, foolishly deliver their own inventions. The meaning is that the beginning of right knowledge is to give the holy prophets the credit that is due to God. He calls them the holy men of God because they faithfully executed the office committed to them, having acted as God's representatives in their ministries. He says that they were—not that they were deprived of their senses (as the Gentiles imagined their prophets to have been), but because they dared not announce anything of their own and obediently followed the Spirit as their guide, who governed their speech as in his own sanctuary. Understand by prophecy of Scripture what is contained in the holy Scriptures.