God created everything that was created in six short days. He could’ve done it much faster, instantaneously, as a matter of fact, but He chose to do it in six days.
One the first day, He created light. Now, the sun, moon, and stars didn’t come until a few days later. One might wonder how there was light without the sun. The sun is a means of light, but God is the source. The Bible speaks of the brightness of God’s glory. When Moses came down from receiving the law at Mt. Siniai, his face had such a glow that he had to put a veil over so the people could look at him. This was because he had been in the presence of God. Revelation 18 tells us of an angel with great authority whose presence illuminates the earth. It is because he comes from the very presence of God. Revelation 21:23 says that in heaven there will be no need of the sun or the moon, because the glory of God lightens it, and the Lamb is the lamp of heaven. So, no problem for God to say, “Let there be light,” and the result to be that there was light, eventhough the sun had not yet been created.
John 1 speaks of Jesus as being the One in whom is life, and that life as being the light of men, which shines in darkness. John says that he is not the light, but that he came as a witness to the light. John 3:19 says, “This is the verdict: that light is come into the world, but men loved darkness instead of light because their deeds were evil.”
The light that God made His first order of business to create, was testimony to His own glory. It was as if He were saying, “All things are about Me.” Romans 11:36 says it like this: “For from Him and through Him and to Him are all things. To Him be the glory forever! Amen.”