Chapter 9: The Atonement of Jesus Christ, B: Because we are fallen, we have need of an Atonement.

We have to have a Savior. We needed to fall to learn good from bad, but we need to be saved to return to God's presence and to continue progressing. Without the Savior's atonement, we would be subjects of the devil for the rest of eternity, endlessly miserable and unhappy.

For as death hath passed upon all men, to fulfill the merciful plan of the great Creator, there must needs be a power of resurrection, and the resurrection must needs come unto man by reason of the fall; and the fall came by reason of transgression; and because man became fallen they were cut off from the presence of the Lord. Wherefore it must needs be an infinite atonement - save it should be an infinite atonement this corruption could not put on incorruption. Wherefore, the first judgment which came upon man must needs have remained to an endless duration. And if so, this flesh must have laid down to rot and to crumble to its mother earth, to rise no more. O the wisdom of God, his mercy and grace! For behold, if the flesh should rise no more our spirits must become subject to that angel who fell from before the presence of the Eternal God, and became the devil, to rise no more. And our spirits must have become like unto him, and we become devils, angels to a devil, to be shut out from the presence of God, and to remain with the father of lies, in misery, like unto himself; yea, to that being who beguiled our first parents, who transformeth himself nigh unto an engel of light, and stirreth up the children of men unto secret combinations of murder and all manner of secret works of darkness. O how great the goodness of our God, who prepareth a way for our escape from the grasp of this awful monster; yea, that monter, death and hell, which I call the death of the body, and also the death of the spirit. And because of the way of deliverance of our God, the Holy One of Israel, this death, of which I have spoken, which is the temporal, shall deliver up its dead; which death is the grave. And this death of which I have spoken, which is the spiritual deah, shall deliver up its dead; which spiritual death is hell; wherefore, death and hell must deliver up their dead, and hell must deliver up its captive spirits, and the grave must deliver up its captive bodies, and the bodies and spirits of men will be restored one to the other; and it is by the power of the resurrection of the Holy One of Israel.
2 Nephi 9:6-12

And now, behold, I will testify unto you of myself that thse things are true. Behold, I say unto you, that I do know that Christ shall come among the children of men, to take upon him transgressions of his people, and that he shall atone for the sins of the world; for the Lord God hath spoken it. For it is expedient that an atonement should be made; for according to the great plan of the Eternal God there must be an atonement made, or else all mankind must unavoidably perish; yea, all are haredened; yea, all are fallen and are lost, and must perish except it be through the atonement which it is expedient should be made.
Alma 34:8-9

All have sinned. Each person is therefore unclean to the extent which he has sinned, and because of that uncleanness is banished from the presence of the Lord so long as the effect of his own wrongdoing is upon him.
Since we suffer this spiritual death as a result of our own trangressions, we cannot claim deliverance therfrom as a matter of justice. Neither has any man the power within himself alone to make restitution so complete that he can be wholly cleansed from the effect of his own wrongdoing. If men are to be freed from the results of their own trangressions and brough back into the presence of God, they must be the benficiaries of some expedient beyond themselves which will free them from the effect of their own sins. For this purpose was the atonement of Jesus Christ conceived and executed.
This was the world's supreme act of charity, performed by Jesus out of his great love for us. He not only thereby met the demands of the law of justice - which would have left us forever marred by the effects of our own trangressions - but made effective the law of mercy, through which all men may be cleansed from their own sins.
Marion G. Romney, in Conference Report, April 1982, 9

To atone is to ransom, reconcile, expiate, redeem, reclaim, absolve, propitiate, make amends, pay the penalty. Thus the atonement of Christ is designed to ransom men from the effect of the fall of Adam in that both spiritual and temporal death are conquered; their lasting effect is nullified. The spiritual death of the fall is replaced by the spiritual life of the atonement, in that all who believe and obey the gospel law gain spiritual or eternal life - life in the presence of God where those who enjoy it are alive to things of righteosness or things of the Spirit. The temporal death of the fall is replaced by the state of immortality which comes because of the atonement and resurrection of our Lord. The body and spirit which separated, incident to what men call the natural death, are reunited in immortality, in an inseperable connection that never again will permit the mortal body to see corruption.
Bruce R. McConkie, Mormon Doctrine, 62