2 Peter 1:21

For prophecy never came by the will of man, etc. They could always foretell the future; but the Spirit itself filled their hearts whenever it wished; thus, it was not in their power to teach whatever they wanted, but they spoke only what they had …

For prophecy came not of old by the will of man - Of any mere man whatever. But the holy men of God - Devoted to him, and set apart by him for that purpose, spake and wrote.

For the prophecy,....

Negatively: It came not by the will of man. Neither the things themselves that are recorded, and make up the several parts of the Old Testament, are the opinions of men, nor was the will of any of the prophets or penmen of the scriptures the rule or reason why any of those things were written which make up the canon of the scripture.

For the prophecy came not in old time - Margin, or, “at any.” The Greek word (ποτὲ pote) will bear either construction. It would be true in either sense, but the reference is particularly to the recorded prophecies in the Old Testament. What was true of them, however, is true of all prophecy, that it is not by the will of man.

"Spake" implies that, both in its original oral announcement, and now even when in writing, it has been always, and is, the living voice of God speaking to us through His inspired servants. Greek, "borne (along)" forms a beautiful antithesis to "was borne." They were passive, rather than active instruments.

For the prophecy came not in old time - That is, in any former time, by the will of man - by a man’s own searching, conjecture, or calculation; but holy men of God - persons separated from the world, and devoted to God’s service, spake, moved by the Holy Ghost. So far were they from inventing these prophetic declarations concerning Christ, or any future event, that they were φερομενοι, carried away, out of themselves and out of the whole region, as it were, of human knowledge and conjecture, by the Holy Ghost, who, without their knowing any thing of the matter, dictated to them what to speak, and what to write; and so far above their knowledge were the words of the prophecy, that they did not even know the intent of those words, but searched what, or what manner of time the Spirit of Christ which was in them did signify, when it testified beforehand the sufferings of Christ, and the glory that should follow. See 1Pe 1:11, 1Pe 1:12, and the notes there. 1. As the writer of this epistle asserts that he was on the holy mount with Christ when he was transfigured, he must be either Peter, James, or John, for there was no other person present on that occasion except Moses and Elijah, in their glorious bodies. The epistle was never attributed to James nor John; but the uninterrupted current, where its Divine inspiration was granted, gave it to Peter alone. See the preface. 2. It is not unfrequent for the writers of the New Testament to draw a comparison between the Mosaic and Christian dispensations; and the comparison generally shows that, glorious as the former was, it had no glory in comparison of the glory that excelleth. St. Peter seems to touch here on the same point; the Mosaic dispensation, with all the light of prophecy by which it was illustrated, was only as a lamp shining in a dark place. […]