The "sure word of prophecy" is the mind of God revealed in the Scripture of truth. This is compared to "a light shining in a dark place." This "dark place" is the heart of man, and a dark place it is; and the light shining in the dark place is when the Spirit of God pours his own heavenly light into the dark heart.
The Spirit of God works by the word of God. He makes use of the Scriptures of truth, by means of these blessed Scriptures to communicate light. There is no light in the Scriptures themselves; they cannot teach a man to profit, that being God's prerogative. They are a dead letter, nothing but a collection of words and syllables; there is no light in them, no, not a particle, but what the Spirit of God throws upon them when he shines through them. I might compare the Scriptures to the moon: the moon has no light in herself, but she borrows all her light from the sun: blot out the sun from the sky, and the moon would cease to shine.
Or I might compare the Scriptures to what James compares them: "If any be a hearer of the word, and not a doer, he is like unto a man beholding his natural face in a glass." Here the Scriptures are compared to a mirror, or looking-glass. But light must shine upon the glass. Of what use is a looking-glass in a dark night? It reflects no image; it presents to you no likeness; you discern not your features therein; it might be nothing else but a naked board, as far as any reflection it gives of your face. But let light come into the room, or let the sun rise and shine upon it, and your countenance is reflected therein.
So with the word of God; it is utterly ineffectual until the Spirit shines upon it; and when he shines upon it, he casts at the same time a ray of light into your heart; and as he shines with this twofold ray, first upon the word, and then into your soul, he reflects from the word your very image, and you see yourself just as you are, clearly portrayed. Now this is the light shining in a dark place; the light of God's truth shining into your dark heart. This becomes "a sure word" to you; faith is raised up in your heart to credit what God has revealed; the shining in of this light into the dark place causes you to believe; and you, believing in the light which is thus come into your dark heart, receive the word of prophecy as a sure word.