Genesis 9:15 And I will remember my covenant, which is between me and you and every living creature of all flesh; and the waters shall no more become a flood to destroy all flesh.
God destroyed the entire world in a flood. Then He made a promise that He would not be doing that again. He gave the rainbow as a symbol of His promise. God keeps His Word and likes to remind us that He does. Every time you see a rainbow, you should remember that God is a God of His Word.
Now the question comes, do you believe God? He is the creator. He is the owner of the entire universe. He is the judge of all His creation. He has called on all to believe Him. He says He will save and forgive sins. He says that He has a better place prepared for us. Can we believe that? Will we believe that?