2 Peter 1:20

Knowing this first. That is, when the Christian comes to study the prophetic word he should constantly keep in mind the basic principle that the apostle now enunciates.

Prophecy of the scripture. Peter here refers to the writings of the OT, possibly distinguishing between the inspired word and the utterances of the false prophets of whom he treats in ch. 2.

Private interpretation. Or, “one’s own interpretation,” referring to the prophet who originally gave the prophecy. The prophet was the Holy Spirit’s mouthpiece, and thus under divine guidance. He was not to intrude his own ideas into the messages that were given him for the benefit of God’s people. The same principle holds true in a study of the prophecies — the reader should strive to understand the Spirit-inspired meaning of the passage under consideration.