We have also a more sure word of prophecy; whereunto ye do well that ye take heed, as unto a light that shineth in a dark place, until the day dawn, and the day star arise in your hearts:
We - all believers.
A more sure, х echomen (G2192) bebaioteron (G949) ton (G3588) profeetikon (G4397) logon (G3056)] - 'we have the word of prophecy more sure' (confirmed); become a firmer ground of confidence. Previously we knew its sureness by faith; through that visible specimen of its future fulfillment, assurance is made doubly sure: so that, even after Peter and the other apostles are dead, those whom he addresses will feel sure that they have "not followed cunningly devised fables." Prophecy assures us that Christ's sufferings, now past, are to be followed by Christ's glory, to come. The transfiguration gives a pledge to make faith still stronger, that "the day" of His glory will "dawn" ere long. He does not mean that the "word of prophecy," or Scripture, is surer than the voice of God at the transfiguration. The fulfillment 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 х luchnoo (G3088), "light"] heeded' by Old Testament believers, until a gleam of the "day dawn" was given at Christ's first coming, 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. Whereunto - to which word of prophecy: primarily the Old Testament (and the New Testament, so far as it was then written) in Peter's day; now also in our day the whole New Testament, which, though brighter than the Old Testament (cf. 1 John 2:8, end), is but a lamp even still, as compared with the brightness of the eternal day (cf. 2 Peter 3:2). Oral teachings and traditions are to be tested by the written word (Acts 17:11). Dark, х auchmeeroo (G850), squalid] - without water, or light. Such spiritually is the world without, and the smaller world (microcosm) within, the natural heart. Compare "dry places," Luke 11:24 (namely, unwatered by the Spirit), through which the unclean spirit goeth.
Dawn - bursting through the darkness. And - and so; namely, by this sample of Christ's glory in His humiliation (John 1:14), and earnest of His coming glory in His exaltation.
Day-star, [fosforos] - "the morning star" (Revelation 22:16); the Lord Jesus. In your hearts. Christ's arising in the heart, by His Spirit giving full assurance, creates spiritually full day, the means to which is prayerfully giving heed to the word. This is associated with the day of the Lord-being the earnest of it. Even our hearts shall not fully realize Christ in all His glory and felt presence, until He shall come (Isaiah 66:14-15; Malachi 4:2). However, Tregelles' punctuation is best, 'Whereunto ye do well to take heed (as unto a light shining in a dark place, until the day have dawned, and the morning star arisen) in your hearts.' For the day has already dawned in the believer's heart; what he waits for is, its visible manifestation at Christ's coming.