2 Peter 1:19

In these words the apostle lays down another argument to prove the truth and reality of the gospel, and intimates that this second proof is more strong and convincing than the former, and more unanswerably makes out that the doctrine of the power and coming of our Lord Jesus Christ is not a mere fable or cunning contrivance of men, but the wise and wonderful counsel of the holy and gracious God. For this is foretold by the prophets and penmen of the Old Testament, who spoke and wrote under the influence and according to the direction of the Spirit of God. Here note, I. The description that is given of the scriptures of the Old Testament: they are called a more sure word of prophecy. 1. It is a prophetical declaration of the power and coming, the Godhead and incarnation, of our Saviour, which we have in the Old Testament. It is there foretold that the seed of the woman shall bruise the serpent's head. His power to destroy the devil and his works, and his being made of a woman, are there foretold; and the great and awful Old Testament name of God, Jehovah (as read by some), signifies only He will be; and that name of God (Exo 3:14) is rendered by many, I will be that I will be; and, thus understood, they point at God's being incarnate in order to the redemption and salvation of his people as what was to come.

— Matthew Henry's Concise Commentary (public domain)

We--all believers. a more sure--rather as Greek, "we have the word of prophecy more sure (confirmed)." Previously we knew its sureness by faith, but, through that visible specimen of its hereafter entire fulfilment, assurance is made doubly sure. Prophecy assures us that Christ's sufferings, now past, are to be followed by Christ's glory, still future: the Transfiguration gives us a pledge to make our faith still stronger, that "the day" of His glory will "dawn" ere long. He does not mean to say that "the word of prophecy," or Scripture, is surer than the voice of God heard at the Transfiguration, as English Version; for this is plainly not the fact. The fulfilment of prophecy so far in Christ's history makes us the surer of what is yet to be fulfilled, His consummated glory. The word was the "lamp (Greek for 'light') heeded" by Old Testament believers, until a gleam of the "day dawn" was given at Christ's first coming, and especially in His Transfiguration. So the word is a lamp to us still, until "the day" burst forth fully at the second coming of "the Sun of righteousness." The day, when it dawns upon you, makes sure the fact that you saw correctly, though indistinctly, the objects revealed by the lamp.

— Jamieson-Fausset-Brown Commentary (public domain)