Jesus was and is the Lamb slain from the foundation of the world (see Revelation 5:6; Revelation 13:8, Moses 7:47). The plan of salvation, which is the gospel of God the Father, was taught and understood in our first estate. There it was decreed that this plan would entail a creation, a fall, and an atonement.
And there it was that Jehovah, the firstborn spirit of the Father, was chosen and foreordained to be the Redeemer and the Savior of all mankind. Joseph Smith spoke of the nature of such premortal agreements:
“Everlasting covenant was made between three personages before the organization of this earth, and relates to their dispensation of things to men on the earth; these personages, according to Abraham’s record, are called God the first, the Creator; God the second, the Redeemer; and God the third, the witness or Testator.”
(Teachings, p. 190)