"And God said, Let there be light: and there was light. And God saw the light, that it was good: and God divided the light from the darkness." (Genesis 1:3-4)
Initially, the created cosmos was in darkness—a darkness which God Himself had to create ("I form the light, and create darkness"—Isaiah 45:7). But then the dark cosmos was energized by the Spirit's moving, and God's light appeared. The darkness was not dispelled, however, but only divided from the light, and the day/night sequence began, which has continued ever since.
This sequence of events in the physical creation is a beautiful type of the spiritual creation, "a new creature" (2 Corinthians 5:17). Each individual is born in spiritual darkness, but "God, who commanded the light to shine out of darkness, hath shined in our hearts, to give the light of the knowledge of the glory of God in the face of Jesus Christ" (2 Corinthians 4:6). We are now "partakers of the inheritance of the saints in light," because He "hath delivered us from the power of darkness" (Colossians 1:12-13).
However, the light in the primeval darkness resulted only in a division of night and day. The night still comes, but God has promised that, in the coming Holy City, "there shall be no night there" (Revelation 22:5).
Just so, even though we have been given a new nature of light, the old nature of darkness is still striving within, and we have to be exhorted: "For ye were sometimes darkness, but now are ye light in the Lord: walk as children of light" (Ephesians 5:8). Nevertheless, "the path of the just is as the shining light, that shineth more and more unto the perfect day" (Proverbs 4:18). When we reach that city of everlasting light, all spiritual darkness will vanish as well, for "there shall in no wise enter into it any thing that defileth" (Revelation 21:27), and we shall be like Christ. HMM