Mormons believe, as did early Christians, that our Father in Heaven is the Father of all spirits. This means you and me and Jesus and Lucifer are brothers. I understand that this idea seems distasteful to many. How could God create something evil? Actually, he did not “create” anyone to be evil, but there are evil people. We have our free-will and it is our choice to be good or evil. Lucifer, son of the morning, chose to rebel against God. He chose to exalt himself above our Eternal Father. He chose to go against God’s plan. In doing so, he became Satan, Fallen Morning Star. This is not something Mormons invented. It is in the ancient Jewish and Christian writings.
Both matter and intelligence are eternal.
Mormons do not believe that we come into existence at the moment of conception; God did not create us out of nothing when the egg and the sperm make contact. God organizes spirits from pre-existing intelligence–intelligence that is eternal. Before our mortal birth, we are spiritually born of God and that spirit is eternal, as our Father is eternal. Jesus was his first born spirit in the pre-mortal life.  Matter, likewise, is organized by God, not created ex nihilo.
Mormons believe that we existed before our mortal birth, that “man was also in the beginning with God.  Intelligence was not created or made, neither can be.” and that Jesus was “the beginning of the creation of God.” (Rev 3:14) This is quite different from modern Christian teachings, but very much in-line with 1st century Christians. For example, Paul taught that all are the off-spring of God. (Acts 17: 22-24, 28-9, Ephesians 4:6, Hebrews 12:9)
Many believed in this doctrine of pre-mortal existence: Justin Martyr, Augustine, Cyril of Jerusalem, Clement of Rome, Origen, Pierius, John of Jerusalem, Rufinus and Nemesius. (footnote 34)

 

Clement records: “For this reason the world has existed through the ages, so that the spirits destine to come here might fulfill their number, and here make their choice between the upper and lower worlds. (footnote 35)

 

Origen taught this in 220 AD: “He described an initial spiritual creation of Heaven and Earth and the spirits of all the later inhabitants of the physical creation. From this spiritual pre-existence, some were born into one of three different glories which Origen called ‘supercelestial, terrestial, and subterrestrial.'” (footnote 36, Origen, “On First Principles”, as quoted in Fair LDS, Michael Hickenbotham)

These early Christian beliefs were condemned later in the Council of Constantipole in 553 AD.



How did Lucifer Become the Devil?
God’s plan is all about choice. To progress, you have to be given freedom–freedom to choose. God’s plan does not include forcing you to choose the right. He does not make sure you choose to keep the commandments. You decide. Unfortunately, some will rebel. Lucifer voiced his plan in opposition to God’s plan. Lucifer presented a plan: not one soul would be lost–he would force everyone to follow God’s commandments. In taking this role, he, Lucifer would become the God of this world. Christ was champion of God’s plan and said the glory would go to the Father. Choice is necessary for our innate intelligence to enlarge, and paramount to God’s plan; some chose to side with Christ, and some with Lucifer. This is when Lucifer became Satan, Fallen Morning Star.

Origen taught that “before the aeons existed, all spirits were pure; demons, souls and angels alike, all served god and did what he commanded them. the devil was one of them. he had free-will and wanted to set himself up against god, but god cast him down.” (footnote 57)

Lucifer (hebrew- original shining one), was in the beginning as “son of the morning”. He chose to exalt himself above God the Father, he rebelled and was cast out because of his choices. He was not created evil, he became evil by what he chose to do.

Isaiah recorded:  How art thou fallen from heaven, O Lucifer, son of the morning! how art thou cut down to the ground, which didst weaken the nations!
For thou hast said in thine heart, I will ascend into heaven, I will exalt my throne above the stars of God: I will sit also upon the mount of the congregation, in the sides of the north:
I will ascend above the heights of the clouds; I will be like the most High.
Yet thou shalt be brought down to hell, to the sides of the pit. (Isaiah 14:12-15)

 

We accept Jesus as God, the only begotten of the Father in the flesh, and first begotten in the spirit. He is our elder brother. We are the offspring of God.
Following are some wonderful illustrations (though, Mormons don’t believe in wings–either for angels or for pre-mortal spirits).

 

Illustrations for John Milton’s “Paradise Lost“ by Gustave Doré, 1866 :
Jesus and God the Father
Satan is cast out the hill of Heaven
The Fall of Lucifer

 

Lucifer is cast out of Heaven

 

Lucifer, cast out, becomes the Devil