"All beings who have bodies have power over those who have not. The devil has no power over us only as we permit him." (Joseph Smith, TPJS, Section 4, p. 181)
"Before foundation of the earth in the Grand Council that the spirits of all men were subject to oppression, and the express purpose of God in giving them a tabernacle was to arm them against the power of Darkness." (Joseph Smith, Discourse, [19 January 1841], as Reported by William P. McIntire's Notebook, p. 7)
"God is good and all his acts are for the benefit of inferior intelligences— God saw that those intelligences had not power to defend themselves against those that had a tabernacle therefore the Lord calls them together in council and agrees to form for them tabernacles so that he might gender the spirit and the tabernacle together so as to create sympathy for their fellow man— for it is a natural thing with those spirits that have the most power to bear down on those of lesser power, so we see the devil is without a tabernacle and the Lord has set bands to all spirits." (Ibid. p. 18)
"And it came to pass that Adam, being tempted of the devil—for, behold, the devil was before Adam, for he rebelled against me, saying, Give me thine honor, which is my power; and also a third part of the hosts of heaven turned he away from me because of their agency; And they were thrust down, and thus came the devil and his angels; And, behold, there is a place prepared for them from the beginning, which place is hell.
"Wherefore, I, the Lord God, caused that he should be cast out from the Garden of Eden, from my presence, because of his transgression, wherein he became spiritually dead, which is the first death, even that same death which is the last death, which is spiritual, which shall be pronounced upon the wicked when I shall say: Depart, ye cursed." (D&C 29:36-38, 41)
"Then shall he say also unto them on the left hand, Depart from me, ye cursed, into everlasting fire, prepared for the devil and his angels." (Matthew 25:41)
Consider that when babies are born, they are Celestial, they are the hosts of heaven. If each of us should consider ourselves Adam and Eve, then who are "before" us which tempt us? And what are they tempting us to stop being like? When we fall, and are "cast out" (Depart, ye cursed), then this earth is a place prepared for the devil and his angels. The goal then, seems to escape from hell.
"O the greatness of the mercy of our God, the Holy One of Israel! For he delivereth his saints from that awful monster the devil, and death, and hell, and that lake of fire and brimstone, which is endless torment." (2 Nephi 9:19)
Satan Fell From Heaven
"Satan was not thrown out of heaven until after this earth was created. Jesus remarked, "I beheld Satan as lightning fall from heaven." (Luke 10:18) Similarly, John's revelation records that Satan "which deceiveth the whole world: he was cast out into the earth, and his angels were cast out with him." (Revelation 12:9) From this we know that Satan was not cast out until after the creation of this earth had prepared a place into which Satan could be cast." (link, emphasis added)