What does Genesis 1:15 mean?

This verse concludes the statement begun in the previous verse. In this passage, God completes His command to create the sun, moon, and stars, on day 4 of creation. In verse 14, God commanded lights to appear in the heaven—meaning the sky. The purpose of these lights, according to God, is to serve several purposes: to separate day from night, to mark the days, years, and seasons and, in this verse, to provide light upon the earth.

This demonstrates the common pattern of Genesis chapter 1. Each of the first three days prepares a setting, while each of the second three days populates that setting. On the first day of creation, God created light. On day four, He creates specific sources of daily and nightly light upon the earth. As mentioned earlier, the idea of light existing prior to the sun and stars seems backwards, but according to modern science it's not so far-fetched.

Regardless of how one reads this verse, it concludes with four unmistakable and remarkable words: "and it was so." This uses the Hebrew phrase wa yehi kēn. This is an almost comically simple way of explaining something as awesome as the creative power of God: He spoke, and the sun, moon, and stars came into being.

The prior verse says God intended our view of the universe to be used for timekeeping. Psalm 19 tells us that the heavens above us serve another purpose: to declare God's glory, to proclaim in what He has made the Creator's magnificence. Both Psalm 19 and Romans 1 insist no language is necessary to see the glory of God from what He has made.