You have heard Christ the Lord preached in these words: “In the beginning was the Word and the Word was with God, and the Word was God” (Jn 1:1). This is Christ the Lord, who if he had not lowered himself but had wished to remain so forever, we would have perished. We acknowledge the Word as God with God; we acknowledge the only Son equal to the Father; we acknowledge the light from light, the day from day. The day who made the day was not himself made by the day but generated from it. If then the day from day was not made but generated, then what is the day the Lord has made? Why is it called day? Because it is light. “And God called the light day.” Let us ask, then, which day the Lord has made that we may rejoice and be glad in it.
At the very beginning of the world’s creation, it is read that “darkness was over the abyss, and the Spirit of God moved over the water. And God said, ‘Let there be light,’ and light was made. And God divided the light from the darkness, and he called the light day and the darkness he called night” (Gen 1:2-5). This is a day the Lord has made. But is it the day in which we are to rejoice and be glad? There is another day the Lord has made which we should instead acknowledge, and rejoice and be glad in it.
It was said to believers in Christ: “You are the light of the world” (Mt 5:14). If a light, then a day, because “God called the light day.” Yesterday God’s Spirit moved over the water here too. There had been darkness over the abyss, because these infants were still bearing their sins. But when through the Spirit their sins were forgiven them, then did the Lord say: “Let there be light, and light was made.” Here is “the day the Lord has made; let us rejoice and be glad in it” (Ps 117, 24).
We are addressing this day with the words of the Apostle: O day which the Lord has made, “you once were darkness, but now you are light in the Lord.” “Once, he says, “you were darkness.” Were you darkness, or not? Recall your deeds, and see if you weren’t. Look at the consciences of yours that you rejected. Because, then, you were once darkness, but now are light, not in yourselves, but in the Lord, “walk like children of the light (Eph 5:8). (Augustine, Sermon 226; PL 38, 1009)