“When I bring clouds over the earth and the bow is seen in the clouds, I will remember my covenant that is between me and you and every living creature of all flesh. And the waters shall never again become a flood to destroy all flesh.” (Genesis 9:14-15)
When the flood is over, Noah and his family are to make a new start. God commands them to be fruitful and multiply and fill the earth — just as the animals are to “swarm on the earth, and be fruitful and multiply on the earth” (Genesis 8:17). This is a repetition of God’s command to Adam and Eve in the creation account. God wants the world to be filled with humans again, like it was in the beginning.
Alongside this commission, God gives a promise. He makes a covenant with “every living creature of all flesh” that He won’t destroy them again by a flood. And He gives a sign to remind humans of this covenant: the rainbow that will appear in the clouds when it rains. So, exactly when “the windows of the heavens are opened” and people may wonder whether the earth will be flooded again, there is this reassuring sign of God’s covenant faithfulness.
This covenant is entirely one-sided. Everyone can trust it, no matter what. God knows that humans will turn out to be evil again, but his promise stands. So, when you see the rainbow, let it remind you of God’s faithfulness!