Your question: why did the gods send the flood to destroy humanity
First of all, there is only ONE God. And only ONE God destroyed humanity during The Flood. Well, with the exception of Noah and his family who built the Ark that carried two of every single animal.
Now to answer your question…
Flood is sent by God to destroy his creation, which has become corrupt and evil
• The humans are so wicked and evil that "it repented the Lord that he had made man on the earth, and it grieved him at his heart" (Gen. 6:6). He says,"I will destroy man whom I have created from the face of the earth. . . " (Gen. 6:7)
Earth once had giants and heroes, but they became part of the evil
• These "giants in the earth" (Gen. 6:4) were the sons of God and of the daughters of men, but they were only men of renown in the old days (Gen. 6:4). There is no Golden Age at the time of the flood.
One good man is saved (Noah), who obeys God's orders to build an ark
• God decides to destroy all but Noah who "found grace in the eyes of the Lord" (Gen. 6:8). He becomes the source of salvation for the creation, a hope for re-creation after the flood. He is also a symbol of God's mercy.
• God orders Noah to build an ark of gopher wood to save himself, his wife, his sons and their wives, along with animals to repopulate the earth. The exact number of animals is unclear--either 2 of each (Gen. 6:19; 7:9) or 7 of each clean beast and fowl, 2 of each unclean beast (Gen. 7:2-3).
A great storm rages and wipes out everyone and everything except the passengers on the ark
• After seven days "the waters of the flood were upon the earth" (Gen.7:10).
• The rain was "upon the earth forty days and forty nights" (Gen. 7:12)
• The flood lasts 150 days (Gen. 7:24) and destroys everything.
• As the waters recede, the ark lands on Mt. Ararat (Gen. 8:4). The other mountain tops appear after approximately 2.5 months more.
After a total of 40 days, Noah releases birds to find land (Gen. 8:5-11):
• raven--sends it out repeatedly, to and fro, looking for land
• dove--sent once and returns sent again after seven days, returns with olive branch, sent again after seven more days, does not return.
Those saved show proper reverence to the gods and are rewarded
• God orders Noah to leave the ark.
• Noah offers a sacrifice to God.
• God makes the first covenant of the Hebrew Bible with Noah.
• God gives the rainbow as a sign that He will never again destroy the earth in a flood.
(Source: http://www.public.iastate.edu/~gbetcher/353/gilgfld.htm , Bible, Common Knowledge)
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