https://www.churchofjesuschrist.org/study/manual/true-to-the-faith/plan-of-salvation
In harmony with the plan of happiness, the premortal Jesus Christ, the Firstborn Son of the Father in the spirit, covenanted to be the Savior. Those who followed Heavenly Father and Jesus Christ were permitted to come to the earth to experience mortality and progress toward eternal life.
Lucifer, another spirit son of God, rebelled against the plan and “sought to destroy the agency of man” He became Satan, and he and his followers were cast out of heaven and denied the privileges of receiving a physical body and experiencing mortality.