"Then God said, "Let there be light"; and there was light." (Gen. 1:3). "For God, who said, "Light shall shine out of darkness," is the One who has shone in our hearts to give the Light of the knowledge of the glory of God in the face of Christ." (2 Cor. 4:6).
While God could have simply thought the physical world into existence, He chose a seven-day framework that shadows the "real" creation through the gospel. It isn't the form (days two and three) or content (days four through six) the Spirit brings first to the soul, but illumination. Until a sinner receives the "light" of the glory of Christ in the gospel, chaos-crushing boundaries and void-destroying life are useless. God's light in creation and recreation comes with three distinguishing marks:
First, it comes sovereignly pronounced -- "Let there be light." No solicitation first by the "waters," no assistance received by the darkness, no gradual development through education. It's spoken and it's done -- complete, total. It's spoken because the Son is the Word (Jn. 1:3; Heb. 1:2). From this light all other "days" will be made -- the entire realm of time and eternity. The light of the gospel once penetrated is never to be rescinded out of the soul -- an eternal day created forever.
Second, it comes judicially established -- "God saw that the light was good and God separated the light from the darkness." God "sees" as a witness, renders a verdict (good) as a judge, then separates as an executioner -- the essence of every legal process. Adam, created in God's image, will be called upon to do the same with the animals (Gen. 2:19). Every sinner turned saint will do the same -- he has the mind of Christ and will render the same verdict as God wherever light and darkness is found (1 Cor. 2:14-16).
Third, it comes eternally memorialized -- "God called the light day." The giving of a name or its interpretation is the essence of control and dominion -- something Adam failed to do with the serpent (Gen. 3) but the second Adam succeeded (Matt. 4:1-11). Light is forever called day -- a demarcation of time for the purpose of work and rest and of serving God (Rom. 13:12-13). But "day" isn't all light -- there is evening, the realm of night -- something God separates but doesn't destroy. It remains, as the sinner is acutely aware, but it's stated first because it will be replaced by morning -- a combination seen only by God's grace as "day."
There's only one outcome when the Spirit who hovers over the water commands light -- His immediate reflection. Something that wasn't there when there was no light. And what is reflected is the blueprint for what is to be created -- and recreated -- in the "days" to come (Rom. 8:29).