IN THE BEGINNING, God creates the Heavens and the Earth. God then creates angels. Lucifer, one of these angels, wants dominion over the Earth, but God creates humans and gives the world to them instead. Unsurprisingly, Lucifer doesn’t like that decision too much. He and a bunch of his angel buddies decide to turn against God in response to this perceived slight. The result is a massive war: Lucifer and All his Angels versus God and all His Holy Hosts of Heaven. (God wins.)
Depending on which source you read, Lucifer either then chooses to leave Heaven after his defeat or he and all his angels are cast out of Heaven by God. Either way, Lucifer is now considered “fallen” and officially becomes Satan, The Adversary. He and the (now) demons reside in Hell from here on out.
God places Adam and Eve, the first man and first woman respectively, in the Garden of Eden, the paradisiacal oasis on the comparatively hostile surface of the Earth. Satan, in the form of the serpent, enters Eden and tempts Eve to do the one thing that God has told her and Adam not to do: eat a fruit from the Tree of Knowledge of Good and Evil. After eating the forbidden fruit herself, Eve then convinces Adam to eat it as well. Because of their sin, Adam and Eve are thrown out of Paradise, and so the journey of God’s Chosen People begins.