The rainbow was given by God as a sign of His mercy upon sinful man. It signified a divine promise that “all flesh shall never again be cut off by the water of the flood, neither shall there be again a flood to destroy the earth” (Genesis 9:11). The rainbow signifies the grace and mercy of a holy God.
Yet, in light of God’s mercy and grace, one must not forget that God is holy and hates sin with a passion. He abhors sin. In fact, it was because of sin that God destroyed the human race (save Noah and his family) with the flood in the first place. His holy anger is stated by God Himself in these words, “I will blot out man whom I have created from the face of the earth” (Genesis 6:7). Why? Because “The Lord saw that the wickedness of man was great on the earth, and that every intent of the thoughts of his heart was only evil continually” (Genesis 6:5). Wickedness is what fills the heart of man—wickedness that has infected all parts of our being, continually—the very wickedness that God hates.