After God flooded the earth because of the wickedness of mankind, He made a covenant with Noah, his descendants, and all of the living creatures on the earth that He would never again destroy the earth with a flood. He put a rainbow in the sky as a sign of His covenant, of His mercy.
In Revelation 4:3, it says that around God's throne is a rainbow. Whatever our God does, He reaches through His mercy to do it. Isn't that amazing to think about? It is not God's will for any to perish, so in His mercy He waits, He woos, He calls, He pleads, He reaches out through mercy to me and to you.