Cause and effect. Spiritually, when we sin, there is a negative effect. It is something that cannot be erased on its own. It is like someone jumping off a cliff. The reality is it is done, you have fallen. In regards to sin, the effects may be fear, darkness, depression, regret, pain. In this mortal fallen mortal world our physical bodies, if we live long enough, will be in a state of decline, decay. Spiritually without some way of erasing the effects of sin, both small and large, our spirits would be in decline and decaying into darkness and misery. In the pre-mortal world we all existed as spirit sons or daughters of God. We are all one big family. God presented a Plan of Salvation, which each of us freely accepted or we wouldn't be on the earth today, where each of us in our second estate would come to this earth, receive a physical body, have our faith tested, and see if we'd choose good or evil and follow God. We would have a veil over our pre-mortal memories. Being born on this earth is a Fall. In it we all will sin once we reach the age of accountability, in one degree or another. But in order to grow spiritually, a fall is necessary. Joy cannot be known if you've never known misery. the same way sweet cannot be tasted and discerned without ever tasting bitter. That a Fall is necessary, and cause and effect must exist for existence, then you have a problem. How to erase the effects of the fall, and free the person from the chains of death, so that they can return to eternal life, a brighter star than ever before with infinite joy. What Heavenly Father presented in His Plan was a Savior. Someone would be needed to be strong and virtuous enough to live a perfect life so that justice would have no chains on them, and then perform an Atonement where that person would literally suffer the eternal consequences of every sin committed by the human family. He or She would feel all the pain, all the regret, all the depression, all the darkness that would have been the end result of all the human family's sins. It would be pain and suffering incomprehensible. It will take infinite mercy to pay the price of infinite justice. Only pure love could accomplish it, any other motive and no one could have had the ability to perform it. When Heavenly Father asked all of us who He should send, the first born of our family,our eldest brother, the great Jehovah, the pre-mortal Jesus, humbly volunteered to perform the Atonement. We all cheered for joy because we had great faith In Jehovah, the pre-mortal Jesus. Heavenly Father accepted this from His most beloved son.
Many Christians may believe the Atonement and the most pain was being nailed on the cross. But I think the bulk if not all of the Atonement was performed in the Garden of Gethsemane. In the space of what appeared to be a few hours to his apostles, Christ somehow suffered the eternal consequences of sin for all of humanity. Time is relative, and so what appeared to be a few hours, to Him it was in reality an eternity. It caused so much much pain and anguish that He bled from every pore. A mere mortal would have passed out and died immediately at the amount of pain involved to sweat one drop of blood, but there was no such relief given. Our mortal minds cannot comprehend completely the depths of the Atonement. That Christ successfully accomplished it, and gave His life, He has risen above all things and has infinite healing power.
The Atonement is the central theme of the entire Biblical religion. The Mormons have received additional revelation on Adam and Eve. The Pearl of Great Price shows that after Adam and Eve left the garden, God commanded them to routinely sacrifice a first born lamb without blemish as an offering. After they had did this they were shown why they were asked to perform these sacrifices. It is symbolic of when Christ would perform the Atonement so that the human family can return to God and live life abundantly. All of the Old Testament prophets knew the real symbolism. The Atonement of Christ is everything. Without it, there would be no resurrection, no life after death. We all would be doomed. That is why whether we know this truth or not in this life, every knee will gladly bow to Christ in the next life and know He is the Savior.