Before we came to this earth, Heavenly Father presented His plan of salvation—a plan to come to earth and receive a body, choose to act between good and evil, and progress to become like Him and live with Him forever.
Our agency—our ability to choose and act for ourselves—was an essential element of this plan. Without agency we would be unable to make right choices and progress. Yet with agency we could make wrong choices, commit sin, and lose the opportunity to be with Heavenly Father again. For this reason a Savior would be provided to suffer for our sins and redeem us if we would repent. By His infinite Atonement, He brought about “the plan of mercy, to appease the demands of justice.”4
After Heavenly Father presented His plan, Lucifer stepped forward, saying, “Send me, … and I will redeem all mankind, that [not even] one soul shall … be lost … ; wherefore give me thine honor.”5 This plan was rejected by our Father, for it would have denied us our agency. Indeed, it was a plan of rebellion.
Then Jesus Christ, Heavenly Father’s “Beloved and Chosen [Son] from the beginning,” exercised His agency to say, “Father, thy will be done, and the glory be thine forever.”6 He would be our Savior—the Savior of the world.
Because of Lucifer’s rebellion, a great spiritual conflict ensued. Each of Heavenly Father’s children had the opportunity to exercise the agency Heavenly Father had given him or her. We chose to have faith in the Savior Jesus Christ—to come unto Him, follow Him, and accept the plan Heavenly Father presented for our sakes. But a third of Heavenly Father’s children did not have faith to follow the Savior and chose to follow Lucifer, or Satan, instead.7
And God said, “Wherefore, because that Satan rebelled against me, and sought to destroy the agency of man, which I, the Lord God, had given him, … I caused that he should be cast down.”8 Those who followed Satan lost the opportunity to receive a mortal body, live on earth, and progress. Because of the way they used their agency, they lost their agency.
Read more about how you are free:
https://www.lds.org/general-conference/2010/10/agency-essential-to-the-plan-of-life?lang=eng