Genesis 1:3

"And God said, Let there be light, and there was light." Genesis 1:3 Immediately there appears the Word, "that true light, which lights man on his coming into the world," John 1:9 and through Him also came light upon the world. From that moment God willed creation to be effected in the Word, Christ being present and ministering unto Him: and so God created. The Word also Himself assume His own form and glorious garb, His own sound and vocal utterance, when God says, "Let there be light." Genesis 1:3 This is the perfect nativity of the Word, when He proceeds forth from God. [Against Praxeas 7,12]

After Moses spoke of heaven and earth, of the darkness, the abyss and the wind that came to be at the beginning of the first night, he then turned to speak about the light that came to be at dawn of the first day. At the end of the twelve hours of that night, the light was created between the clouds and the waters and it chased away the shadow of the clouds that were overshadowing the waters and making them dark. For Nisan was the first month; in it the number of the hours of day and night were equal. The light, then, remained a length of twelve hours so that each day might also obtain its [ own ] hours just as the night possesses a measured length of time. Although the light and the clouds were created in the twinkling of an eye, the day and the night of the first day were each completed in twelve hours.
The light then was like a bright mist over the face of the earth. Whether it was like the dawn or like the pillar that gave light in the wilderness to the people, it is obvious that it was unable to chase away the darkness that was spread over the face of everything, unless it had spread out completely over everything, either by its substance or by its appearance. The light was released so that it might spread over everything without being fastened down. It dispersed the darkness that was over everything although it did not move. It was only when [ the light ] went away and when it came that it moved, so that when [ the light ] went away the rule was given to the night and at [ the light's ] coming there would be an end to [ the night's ] rule.

The light was released so that it might spread over everything without being fastened down. It dispersed the darkness that was over everything although it did not move. It was only when [the light] went away and when it came that it moved, for when [the light] went away the rule was given to the night, and at [the light’s] coming there would be an end to [the night’s] rule. After the brightness [of the light] rendered its service for three days … the sun was in the firmament in order to ripen whatever had sprouted under that first light.

The first word of God created the nature of light; it made darkness vanish, dispelled gloom, illuminated the world, and gave to all beings at the same time a sweet and gracious aspect. The heavens, until then enveloped in darkness, appeared with that beauty which they still present to our eyes. The air was lighted up, or rather made the light circulate mixed with its substance, and, distributing its splendour rapidly in every direction, so dispersed itself to its extreme limits. Up it sprang to the very æther and heaven. In an instant it lighted up the whole extent of the world, the North and the South, the East and the West. For the æther also is such a subtle substance and so transparent that it needs not the space of a moment for light to pass through it. Just as it carries our sight instantaneously to the object of vision, so without the least interval, with a rapidity that thought cannot conceive, it receives these rays of light in its uttermost limits. With light the æther becomes more pleasing and the waters more limpid. These last, not content with receiving its splendour, return it by the reflection of light and in all directions send forth quivering flashes. The divine word gives every object a more cheerful and a more attractive appearance, just as when men in deep sea pour in oil they make the place about them clear. So, with a single word and in one instant, the Creator of all things gave the boon of light to the world.

Let there be light. The order was itself an operation, and a state of things was brought into being, than which man's mind cannot even imagine a pleasanter one for our enjoyment. It must be well understood that when we speak of the voice, of the word, of the command of God, this divine language does not mean to us a sound which escapes from the organs of speech, a collision of air struck by the tongue; it is a simple sign of the will of God, and, if we give it the form of an order, it is only the better to impress the souls whom we instruct.

The first word of God created the nature of light; it made darkness vanish, dispelled gloom, illuminated the world, and gave to all beings at the same time a sweet and gracious aspect. The heavens, until then enveloped in darkness, appeared with that beauty which they still present to our eyes. The air was lighted up, or rather made the light circulate mixed with its substance, and, distributing its splendour rapidly in every direction, so dispersed itself to its extreme limits. Up it sprang to the very æther and heaven. In an instant it lighted up the whole extent of the world, the North and the South, the East and the West. For the æther also is such a subtle substance and so transparent that it needs not the space of a moment for light to pass through it. Just as it carries our sight instantaneously to the object of vision, so without the least interval, with a rapidity that thought cannot conceive, it receives these rays of light in its uttermost limits. With light the æther becomes more pleasing and the waters more limpid. These last, not content with receiving its splendour, return it by the reflection of light and in all directions send forth quivering flashes. The divine word gives every object a more cheerful and a more attractive appearance, just as when men in deep sea pour in oil they make the place about them clear. So, with a single word and in one instant, the Creator of all things gave the boon of light to the world.
Let there be light. The order was itself an operation, and a state of things was brought into being, than which man's mind cannot even imagine a pleasanter one for our enjoyment. It must be well understood that when we speak of the voice, of the word, of the command of God, this divine language does not mean to us a sound which escapes from the organs of speech, a collision of air struck by the tongue; it is a simple sign of the will of God, and, if we give it the form of an order, it is only the better to impress the souls whom we instruct.

God is the author of light, and the place and cause of darkness is the world. But the good Author uttered the word light so that he might reveal the world by infusing brightness therein and thus make its aspect beautiful. Suddenly then, the air became bright and darkness shrank in terror from the brilliance of the novel brightness.

As the words themselves make sufficiently clear, we are told that this light was made. The light born from God is one thing; the light that God made is another. The light born from God is the very Wisdom of God, but the light made by God is something mutable, whether corporeal or incorporeal.

We ought to understand that God did not say “Let there be light” by a sound brought forth from the lungs or by the tongue and teeth. Such thoughts are those of persons physically preoccupied. To be wise in accord with the flesh is death. “Let there be light” was spoken ineffably.

And God said, "Let there be light," and there was light. It is fitting for the works of God that the adornment of the world should begin with light: for since He Himself is the true light and dwells in inaccessible light, whose most blessed vision the angels in the heavens of heavens began to enjoy immediately after they were created, He also suitably granted to this world the first grace of material light to be its adornment, so that the other things He created might appear from the source of that light. However, that God is said to have spoken, whether for light to come into being or for other things, we must not believe to be by a bodily voice as we do, but rather that it is understood higher that God spoke for creation to be made, because through His Word He made all, that is, through His only-begotten Son: about whom the Evangelist John speaks more plainly: "In the beginning," he says, "was the Word, and the Word was with God, and the Word was God. It was in the beginning with God; all things were made through Him" (John 1:1). Therefore, what John says, all things were made through the Word of God, is what Moses says, that God said: "Let there be light"; said: "Let there be a firmament"; said: "Let there be," and other creatures. This the psalm also says with the addition of the Holy Spirit's person: "By the word of the Lord were the heavens established and all their host by the spirit of His mouth" (Psalm 33:6). But if it is asked in what places light was made on God's command, while the abyss still covered the entire breadth of the earth, it is undoubtedly clear that it shone in the higher parts of that same earth, which the daylight of the sun now usually illuminates. Nor should it be surprising to us that light can shine in the waters by divine operation, since it is established that waters are often illuminated even by human operation, particularly by sailors, who, submerged in the depths of the sea, make them transparent and clear by emitting oil from their mouths. For if man can do such things with the oil from his mouth, how much more must we believe God can create through the Spirit of His mouth, especially as we must believe much rarer waters existed in the beginning than we now usually see on earth, before they were gathered together into one place so that dry land might appear.

Be light made: I answer, then, with Dionysius (Div. Nom. iv), that the light was the sun's light, formless as yet, being already the solar substance, and possessing illuminative power in a general way, to which was afterwards added the special and determinative power required to produce determinate effects. Thus, then, in the production of this light a triple distinction was made between light and darkness. First, as to the cause, forasmuch as in the substance of the sun we have the cause of light, and in the opaque nature of the earth the cause of darkness. Secondly, as to place, for in one hemisphere there was light, in the other darkness. Thirdly, as to time; because there was light for one and darkness for another in the same hemisphere; and this is signified by the words, "He called the light day, and the darkness night."
Augustine seems to say (De Civ. Dei xi, 9,33) that Moses could not have fittingly passed over the production of the spiritual creature, and therefore when we read, "In the beginning God created heaven and earth," a spiritual nature as yet formless is to be understood by the word "heaven," and formless matter of the corporeal creature by the word "earth." And spiritual nature was formed first, as being of higher dignity than corporeal. The forming, therefore, of this spiritual nature is signified by the production of light, that is to say, of spiritual light. For a spiritual nature receives its form by the enlightenment whereby it is led to adhere to the Word of God.