Before I was born, I lived with my Heavenly Father as His spirit child. In the Council in Heaven, the Father presented His plan of salvation to all His spirit children. Jesus Christ was chosen to be our Savior. Satan sought to amend the plan, taking away free agency; he and his followers were cast out. All who do, have, or will live upon the Earth accepted Heavenly Father’s plan and chose to follow Jesus Christ. Paraphrased from Come Follow Me, February 2014.
Revelation 12:7–9, 11, in the premortal life, we overcame Satan by our testimonies of Jesus Christ.
D&C 138:55–56; Abraham 3:22–26, noble spirits in premortal life were chosen and prepared to do God’s work in this life.
Moses 4:1–4, Satan, the father of all lies, seeks to destroy the agency of man and was cast out.
I remember learning this, when the missionaries taught me 37 years ago. I always wondered what the deal was with Satan. Why was there an adversary on Earth that delighted in getting me to choose wrongly? Finally, it made sense. Jealousy and pride make us blind to correct choices. As an adult, I read a dramatization of the battle the adversary and his angels wage against individuals, The Screwtape Letters. It was chilling but very eye opening. In fact, I think I'll read it again soon.
The idea that each person has a specific God-given mission to discover and perform has inspired me (or put a fire under me) to always find ways to make the world better, to find ways to serve God's children, to find ways to love others as He loves me. Yes, I fail again and again. However, the point isn't our necessarily our success but our willingness to try.