Why is the atonement necessary?
Okay, I’ve been avoiding answering this question – only because it is such a massive topic!
But my procrastination has resulted in my guilt. I should just look directly to the scriptures, and start there.
For I know that ye have searched much, many of you, to know of things to come; wherefore I know that ye know that our flesh must waste away and die; nevertheless, in our bodies we shall see God.
For as death hath passed upon all men, to fulfil the merciful plan of the great Creator, there must needs be a power of resurrection, and the resurrection must needs comes 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… (2 Nephi 9: 4, 6)
We are stopped from returning to God’s presence by two deathly obstacles: physical death and spiritual death.
“Physical death is the separation of the spirit from the physical body. Because of the Fall of Adam, all mankind will suffer physical death.”  Spiritual death when someone is “cut off from the presence of the Lord” (Alma 42:9). Both these deaths are a result from the Fall of Adam and Eve.
That is, because Adam and Eve (and thus all mankind) were literally kicked out of the presence of God (in the Garden of Eden), we are separated spiritually from God. And we are also mortal, and will die, so we are physically separated from God also.
All of God’s children who lived in mortality will overcome physical and spiritual death through the powers of the Atonement of Jesus Christ. This principle was also taught later in the Book of Mormon by Samuel the Lamanite (see Helaman 14:17). In fact, the Book of Mormon is filled with scriptures proclaiming the Saviour’s atoning mission. 
Jesus condescended (lowered himself) to live on Earth and be subject to ridicule, torture. He took upon Himself all the pains and sufferings of all mankind. It took place in the Garden of Gethsemane, and it was very real. He was later crucified, but rose on the third day (the Resurrection), overcoming both spiritual death and physical death for us. This is what we call the Atonement.
The Atonement is central to God’s merciful plan for us. It basically ransoms all mankind from the Fall (overcoming spiritual death). Jesus, as our Saviour, will pay the price necessary for justice to be carried out, and we will all be saved from death, hell, the devil etc. (overcoming physical death).
The leaders of my church - the First Presidency and the Quorum of the Twelve Apostles – have declared to the world the central role of the Saviour and His influence on all mankind:
“We offer our testimony of the reality of His matchless life and the infinite virtue of His great atoning sacrifice. None other has had so profound an influence upon all who have lived and will yet live upon the earth.
“He was the Great Jehovah of the Old Testament, the Messiah of the New. . . .
“He instituted the sacrament as a reminder of His great atoning sacrifice. He was arrested and condemned on spurious charges, convicted to satisfy a mob, and sentenced to die on Calvary’s cross. He gave His life to atone for the sins of all mankind. His was a great vicarious gift in behalf of all who would ever live upon the earth.
“We solemnly testify that His life, which is central to all human history, neither began in Bethlehem nor concluded on Calvary. He was the Firstborn of the Father, the Only Begotten Son in the flesh, the Redeemer of the world”. 
“Our salvation depends on believing in and accepting the Atonement. Such acceptance requires a continual effort to understand it more fully. The Atonement advances our mortal course of learning by making it possible for our natures to become perfect. . . .
“. . . Any increase in our understanding of His atoning sacrifice draws us closer to Him. Literally, the Atonement means to be ‘at one’ with Him. The nature of the Atonement and its effects is so infinite, so unfathomable, and so profound that it lies beyond the knowledge and comprehension of mortal man. . . .
“We long for the ultimate blessing of the Atonement—to become one with Him, to be in His divine presence, to be called individually by name as He warmly welcomes us home with a radiant smile, beckoning us with open arms to be enfolded in His boundless love. How gloriously sublime this experience will be if we can feel worthy enough to be in His presence! The free gift of His great atoning sacrifice for each of us is the only way we can be exalted enough to stand before Him and see Him face-to-face. The overwhelming message of the Atonement is the perfect love the Savior has for each and all of us. It is a love which is full of mercy, patience, grace, equity, long-suffering, and, above all, forgiving.” 
There is no need to worry. The scope of Christ’s Atonement is infinite and eternal. Literally. “Its effects cover all men, the earth itself and all forms of life thereon, and reach out into the endless expanses of eternity.”  It is without end. And it is infinite beyond human comprehension.
The Book of Mormon offers us clarity and comfort:
And now, behold, I will testify unto you of myself that these 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 the 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 hardened; yea, all are fallen and are lost, and must perish except it be through the atonement which it is expedient should be made.
For it is expedient that there should be a great and last sacrifice; yea, not a sacrifice of man, neither of beast, neither of any manner of fowl; for it shall not be a human sacrifice; but it must be an infinite and eternal sacrifice.
Now there is not any man that can sacrifice his own blood which will atone for the sins of another. Now, if a man murdereth, behold will our law, which is just, take the life of his brother? I say unto you, Nay.
But the law requireth the life of him who hath murdered; therefore there can be nothing which is short of an infinite atonement which will suffice for the sins of the world.
Therefore, it is expedient that there should be a great and last sacrifice, and then shall there be, or it is expedient there should be, a stop to the shedding of blood; then shall the claw of Moses be fulfilled; yea, it shall be all fulfilled, every jot and tittle, and none shall have passed away.
And behold, this is the whole meaning of the law, every whit pointing to that great and last sacrifice; and that great and last sacrifice will be the Son of God, yea infinite and eternal.
And thus he shall bring salvation to all those who shall believe on his name; this being the intent of this last sacrifice, to bring about the bowels of mercy, which overpowereth justice, and bringeth about means unto men that they may have faith unto repentance.
And thus mercy can satisfy the demands of justice, and encircles them in the arms of safety, while he that exercises no faith unto repentance is exposed to the whole law of the demands of justice; therefore only unto him that has faith unto repentance is brought about the great and eternal plan of redemption.
Therefore may God grant unto you, my brethren, that ye may begin to exercise your faith unto repentance, that ye begin to call upon his holy name, that he would have mercy upon you;
O how great the plan of our God!
“If physical death should strike before moral wrongs have been made right, opportunity for repentance will have been forfeited. Thus, ‘the [real] sting of death is sin’ (1 Corinthians 15:56).
“Even the Savior cannot save us in our sins. He will redeem us from our sins, but only upon condition of our repentance. We are responsible for our own spiritual survival or death (see Romans 8:13–14; Helaman 14:18; D&C 29:41–45).” 
This leads beautifully into my next question that the Book of Mormon can help answer…
If you wish to order a free copy, let me know or click here.
Feel free to read or listen to it here.
 Earl C. Tingey, Ensign, May 2006, 73
 see 2 Nephi 2:9–10; 9:15, 22, 38; Alma 11:43–44; 12:12–15, 24; 42:23; 3 Nephi 26:4
 “The Living Christ: The Testimony of the Apostles,” Ensign, Apr. 2000, 2–3
 James E. Faust, Ensign, Nov. 2001, 18, 20
 Bruce R. McConkie, Mormon Doctrine, 2nd ed. , 64; see also Moses 7:30
 Russell M. Nelson, Ensign, May 1992, 73