We believe that through the Atonement of Christ, all mankind may be saved, by obedience to the laws and ordinances of the Gospel.
The Atonement of Jesus Christ is the central tenet of our faith. The word ‘atonement’ or ‘at-one-ment’ was done to bring us to be “at one” with God. In other words, we can be reconciled with God through the atonement. The reason we are estranged from Him in the first place is because of our mortality. Because we are mortal, we will all die because our bodies are imperfect. Not only that, our spirits are imperfect. No matter what we do, on our own, we cannot remove past disobedience from our heart. These imperfections of body and spirit bring on physical and spiritual death. Physical death is a separation from our body and spiritual death is a separation from God.
Through the atonement, our bodies will be made perfect after this life, that much is guaranteed to us. In order to perfect our spirits, we must accept the terms and conditions set by Jesus Christ. The atonement was accomplished by Jesus Christ by His suffering for our sins in Gethsemane and on the cross and through His resurrection.
See also:
What do Mormons believe about the Atonement of Jesus Christ?