Most people have heard the Biblical story of Noah's Flood. It's painted on nearly every nursery wall in every church one is likely to visit, in fact. (Why such a non-child-friendly story has become synonymous with nursery decor is a whole other issue for another time, though!)
While most people know the general outline of the story, as is often the case with famous Biblical accounts, the details often get blurred, ignored or "fable-ized" in order to present a nice neat bedtime story for children. The story of Noah though actually spans three chapters of the book of Genesis and is the longest account in this "preface" to the Bible (which is how Genesis 1-11 essentially functions).
It is the distillation by Moses--according to traditional views of Torah's authorship--of the events surrounding the great flood in the ancient world, which also found its way in various forms into nearly every other literary account from that part of the world. The idea of a cataclysmic flood nearly wiping out humanity resides in the collective memory of many civilizations, which argues strongly for an historical event which was interpreted in a variety of ways by subsequent societies that arose after it (for the Biblical account of these peoples' spread and development after the flood, see the "Table of Nations" in Genesis 10).
One particular detail of the Flood story that has always been acknowledged, but frequently misunderstood, is the appearance of the rainbow after the waters subsided. It is commonly assumed by readers that the Noah story is telling us where rainbows come from. That it is an example of biblical etiology. That is, rainbows were created by God after the Flood and that's why they appear now in the clouds at times. Though there is a kernel of truth to this, it isn't what the text itself says.
Here is the passage in question:
Then God said to Noah and to his sons with him: "I now establish my covenant with you and with your descendants after you and with every living creature that was with you--the birds, the livestock and all the wild animals, all those that came out of the ark with you--every living creature on earth. I establish my covenant with you: Never again will all life be cut off by the waters of a flood; never again will there be a flood to destroy the earth."
And God said, "This is the sign of the covenant I am making between me and you and every living creature with you, a covenant for all generations to come: I have set my rainbow in the clouds, and it will be the sign of the covenant between me and the earth. Whenever I bring clouds over the earth and the rainbow appears in the clouds, I will remember my covenant between me and you and all living creatures of every kind. Never again will the waters become a flood to destroy all life. Whenever the rainbow appears in the clouds, I will see it and remember the everlasting covenant between God and all living creatures of every kind on the earth."
So God said to Noah, "This is the sign of the covenant I have established between me and all life on the earth."
(Genesis 9:8-17 NIV)
The nature of this text is somewhat hymnic and repetitive, in accordance with ancient Near East covenant treaty language (which, rather than a bedtime story, is what it actually is!). It is also filled with meaning and symbolism that ancient readers/hearers would've picked up on, but which modern readers often miss entirely.
In the ancient world, the waters/sea represented the epitome of evil, chaotic, malevolent spiritual forces. The waters were personified in various ways throughout the ancient Near East, most often by a dragon or serpent (variously named "Rahab", "Tiamat", "Lotan/Leviathan", etc.) with whom the god or gods must battle and subdue in order to create the universe. Scripture later picks up on this motif and uses it to describe not just God's work at Creation, but also his deliverance of Israel from Egypt through the Reed Sea at the Exodus (i.e. Job 26:12, Psalm 89:10, Isaiah 51:9).
Thus God's use of the abysmal Flood waters in purging the land of sinful humanity would speak volumes to the ancient audience regarding his sovereignty over this most feared and uncontrollable aspect of creation.
But if God unleashed the Deep over the face of the land once, what is to prevent him from doing it again should humanity devolve to equal or worse depths of sin? This is the question that would've been in the mind of Noah and his descendents.
To assure them that he would keep his word, God tells them he has made a covenant with all life on the land. A covenant is a binding agreement that has spiritual, legal and ethical foundations. It is a solemn oath. Covenants were often accompanied by a sign or memorial monument of some sort which would stand as continual witnesses to the original proclamation and remind all onlookers of its ongoing effects. The sign of a covenant was often an ordinary aspect of nature or cultural practice that was set apart and instilled with new significance. And in this instance, God instills a natural phenomenon--the rainbow--with covenantal significance.
Nothing is said of God "creating" rainbows here. Nor is there any implication that before the flood, light did not refract across the visible spectrum when passing through atmospheric water droplets (which is what a rainbow is of course)! Rather, just as God would later take a commonly-practiced cultural ritual--circumcision--and instill it with new covenant meaning for Abraham and his descendents (Genesis 17:10-14), God's "setting his bow in the clouds" was his way of instilling spiritual significance into what had previously been merely a natural phenomenon.
But what is the spiritual or theological significance of a rainbow?
This is where historical background of the biblical world comes into play.
Interestingly, the Hebrew word is simply "bow"...as in bow-and-arrow. And in the world of the 2nd millennium ancient Near East, there was great significance in a conquering lord's bow and how it was displayed. It had potential to be a fearsome and dreadful omen for a defeated and subdued people. Or it could be a promise of peace and benevolence on the part of their ruler. As ancient Near East and Old Testament scholar Meredith Kline put it:
There are Near Eastern representations of kings, first seen engaged in battle, then returning in peace, with the state-god of the storm depicted above in stance identical to the king's in each case. In the battle scene king and god hold bows fitted with arros and full drawn, while in the peace scene their bows hang at their side, loosened. Accordingly, the designation of the rainbow as a battle-bow may best be interpreted as suggesting the picture of the divine warrior with his weapons laid aside, turning from the path of judgment against rebellious mankind, preapred now to govern them with forbearance for a season.
Kline, Kingdom Prologue, p.248
Rather than being an angry storm god who draws his bow for war whenever the storm clouds appear (lightning in the ancient world was often described as the gods' "arrows" in fact), Yahweh is communicating powerfully to his people--in terms they readily would understand and appreciate--that though the heavenly "bow" may appear, it is hung in peace rather than drawn for battle. He will not flood the land again. The bow is a public sign for all to see--including God himself, and thus by extension fearful humanity--that whatever future judgment of God's enemies may consist of, it won't come in the form of the great Flood.
This is the theological message communicated by the rainbow. Sadly, it's rarely preached in the pulpits or Sunday School classes of most churches. But it's a powerful and comforting truth coming on the heels of a tragic and terrible story in the ancient library known as the Bible.
For more on reading Scripture in light of its ancient Near East setting rather than modern scientific assumptions see the author's DVD "The Bible & Science: Friends or Foes??"