From the additional notes to Charles Spurgeon's The Treasury of David, on Psalm 118.
Verse 27. God is the LORD, which hath shewed us light. The Psalmist was clearly possessed of light, for he says, "God is the Lord, which hath shewed us light." He was evidently, then, possessed of light; and this light was in him as "the light of life." This light had shone into his heart; the rays and beams of divine truth had penetrated into his conscience. He carried about with him a light which had come from God; in this light he saw light, and in this light he discerned everything which the light manifested. Thus by this internal light he knew what was good and what was evil, what was Sweet and what was bitter, what was true and what was false, what was spiritual and what was natural. He did not say, This light came from creature exertion, this light was the produce of my own wisdom, this light was nature transmuted some action of my own will, and thus gradually rose into existence from long time and assiduous cultivation. But he ascribes the whole of that light which he possessed unto God the Lord, as the sole author and the only giver of it.
Now, if God the Lord has ever showed you and me the same light which he showed his servant of old, we carry about with us more or less of a solemn conviction that we have received this light from him. There will, indeed, be many clouds of darkness to cover it; there will often be doubts and fears, hovering like mists and fogs over our souls, whether the light which we have received be from God or not. But in solemn moments when the Lord is pleased a little to revive his work; at times and seasons when he condescends to draw forth the affections of our hearts unto himself, to bring us into his presence, to hide us in some measure in the hollow of his hand, and give us access unto himself, at such moments and seasons we carry about with us, in spite of all our unbelief, in spite of all the suggestions of the enemy, in spite of all doubts and fears and suspicions that rise from the depths of the carnal mind, in spite of all these counter workings and undermining, we carry about with us at these times a solemn conviction that we have light, and that this light we have received from God. And why so? Because we can look back to a time when we walked in no such light, when we felt no such light, when everything spiritual and heavenly was dark to us, and we were dark to them.