God saw that the light was good, and he separated the light from the darkness.
Genesis 1:4 (NIV)
In Genesis 1:4, it is God who separates light from darkness.
God made two great lights—the greater light to govern the day and the lesser light to govern the night. He also made the stars. God set them in the vault of the sky to give light on the earth, to govern the day and the night, and to separate light from darkness. And God saw that it was good.
Genesis 1:16-18 (NIV)
But in Genesis 1:16-18, the ongoing responsibility of keeping darkness separate from light is given to the two great lights and the stars in the sky.
We are the light of the world. It is also our responsibility.