Humans Accepted Savior’s Plan in Pre-existence

Moses 4:1-4

1 And I, the Lord God, spake unto Moses, saying: That Satan, whom thou hast commanded in the name of mine Only Begotten, is the same which was from the beginning, and he came before me, saying—Behold, here am I, send me, I will be thy son, and I will redeem all mankind, that one soul shall not be lost, and surely I will do it; wherefore give me thine honor.

2 But, behold, my Beloved Son, which was my Beloved and Chosen from the beginning, said unto me—Father, thy will be done, and the glory be thine forever.

3 Wherefore, because that Satan rebelled against me, and sought to destroy the agency of man, which I, the Lord God, had given him, and also, that I should give unto him mine own power; by the power of mine Only Begotten, I caused that he should be cast down;

Encyclopedia of Mormonism – Plan of Salvation – The Premortal Existence

A Council in Heaven was held of all the spirits, and two individuals volunteered to serve as the redeemer. One was Lucifer, a son of the morning (Isa. 14:12; D&C 76:26), who said he would "redeem all mankind, that one soul shall not be lost," but they would have no choice in the matter. Their agency would be destroyed (Moses 4:1-3). Such a proposal was out of harmony with the plan of the Father, for the agency of mankind is an absolute prerequisite to progress. Jehovah, the premortal Jesus Christ, had first stepped forward and volunteered to give his life as payment for all sins. He set no plan or conditions of his own, but said, "Father, thy will be done, and the glory be thine forever" (Moses 4:2). He was selected by the Father.