8 How shall I make those whose minds are full of vanity understand that the actions of the prophets were also mystical and prophetic? The vanity of their minds is shown in their thinking that we believe God to have once existed in darkness, because it is written, "Darkness was over the deep." As if we called the deep God, where there was darkness, because the light did not exist there before God made it by His word. From their not distinguishing between the light which is God, and the light which God made, they imagine that God must have been in darkness before He made light, because darkness was over the deep before God said, "Let there be light, and there was light."
In the New Testament both these things are ascribed to God. For we read, "God is light, and in Him is no darkness at all;" and again, "God, who commanded the light to shine out of darkness, has shined in our hearts." So also, in the Old Testament, the name "Brightness of eternal light" is given to the wisdom of God, which certainly was not created, for by it all things were made; and of the light which exists only as the production of this wisdom it is said, "You will light my candle, O Lord; my God, You will enlighten my darkness." In the same way, in the beginning, when darkness was over the deep, God said, "Let there be light, and there was light," which only the light-giving light, which is God Himself, could have made.
Source: Reply to Faustus the Manichaean (New Advent)