what events occurred in heaven prior to satans leaving

Book I of Paradise Lost begins with Milton describing what he intends to undertake with his epic: the story of Man's first disobedience and the "loss of Eden," subjects which have been "unattempted yet in prose or rhyme." His main objective, however, is to "justify the ways of God to men." There was a power struggle of sorts. Satan once an archangel apperently wanted a better deal. He is cast into a fiery lake which sets the story which casts Satan a rebel.

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because god is more powerful than satan. God is true and does good for humans.