After God flooded the world and redeemed creation, He set a sign in the skies as a reminder of His promise that He would never do that again - the rainbow.
We are sometimes tempted to think that what God is doing is secret, that we aren't supposed to know. That, perhaps, we can't know. There are even some very popular teachings that espouse this; pastors preach it from the pulpit. God is so much bigger, so much greater, so much more powerful than we could possibly imagine, and we can't figure Him out if we try, and we aren't even supposed to try. After all, if we know, then what is faith?
But faith isn't blind. Not true faith. Faith knows and trusts the one it believes in, and how can we know and trust unless we can see? This is one of the things that I love about God - He doesn't hide Himself. He doesn't do things and tell us to "just deal with it." He doesn't use his big-ness, His great-ness, His power to steamroll over His creation and do whatever He wants without letting us in on it. In fact, the Bible is the grand story of just how in on it we are.
And this rainbow reminds us. It reminds us that God has promised He will never again destroy His creation like He did in the early chapters of Genesis, but it's more than that. This rainbow reminds us that God paints His promises into creation itself so that they aren't a secret, so that we're not left wondering what God is doing, what He's thinking, what He's planning. He's planning our redemption. He's thinking about His love for us. He's giving us His promise over and over again, every time the light dances on the water in the sky that was once formless and void.