Spiritual death is separation from God. The scriptures teach of two sources of spiritual death. The first source is the Fall, and the second is our own disobedience. Spiritual death can be overcome through the Atonement of Jesus Christ and by obedience to His gospel.
The Book of Mormon prophet Samuel taught, "All mankind, by the fall of Adam being cut off from the presence of the Lord, are considered as dead, both as to things temporal and to things spiritual" (Helaman 14:16). During our life on the earth, we are separated from God's presence. Through the Atonement, Jesus Christ redeems everyone from this spiritual death. Samuel testified that the Savior's Resurrection "redeemeth all mankind from the first death—that spiritual death. . . . Behold, the resurrection of Christ redeemeth mankind, yea, even all mankind, and bringeth them back into the presence of the Lord" (Helaman 14:16–17). The prophet Lehi taught that because of the Atonement, "all men come unto God; wherefore, they stand in the presence of him, to be judged of him according to the truth and holiness which is in him" (2 Nephi 2:10).
Further spiritual death comes as a result of our own disobedience. Our sins make us unclean and unable to dwell in the presence of God (see Romans 3:23; Alma 12:12-16, 32; Helaman 14:18; Moses 6:57). Through the Atonement, Jesus Christ offers redemption from this spiritual death, but only when we exercise faith in Him, repent of our sins, and obey the principles and ordinances of the gospel (see Alma 13:27–30; Helaman 14:19; Articles of Faith 1:3).
—See True to the Faith (2004), 48
"Doors of Death"
Russell M. Nelson, Ensign, May 1992, 72–74
Spiritual death is more likely when goals are unbalanced toward things physical.
"The Fall of Man and His Redemption"
Gerald N. Lund, Ensign, Jan. 1990, 22–27
Adam and Eve didn't catch heaven by surprise. The Fall and Redemption had always been part of the plan.
"The Light and Life of the World"
Dallin H. Oaks, Ensign, Nov. 1987, 63–66
The Lord Jesus Christ, our Savior and our Redeemer, is the life of the world because His Resurrection and His Atonement save us from both physical and spiritual death.
Guide to the Scriptures
"Jesus Christ Redeems the Repentant from Spiritual Death"
Teachings of Presidents of the Church: Joseph F. Smith, Chapter 11
Gospel Principles, Chapter 12