JOEL - I have often wondered about this myself. Who's idea was this anyway that someone had to pay for our sins by suffering and dying for us, and how does just one person doing that pay for everyone's sins?
The only thing I can think of is that concepts such as the atonement, good and evil, and punnishment for sins are things that must be considered in terms of infinite application. In other words, they are eternal laws. They have no beginning and no end. This plan of salvation we live has been going on in the eternal past in an eternal number of universes and will continue on in the eternal future. Or in other, other words, your question is impossible to answer, because there is no answer. We just have to accept the fact that it exists and that it is something we have to believe in in order to gain eternal life.
James E. Faust once said,
"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." (James E. Faust, “The Atonement: Our Greatest Hope,” Ensign, Nov. 2001, 18)
The sinless Jesus paying for our sins by suffering and dying for us, is governed by eternal laws. Because these laws are eternal we can not ask why they exist. To do so would imply that someone somewhere made a decision to make up these laws; but that is not what happened.
The fall of Adam brought sin into the world. It is an eternal law that justice requires payment for sin, including even repented sins. They must be paid by the sinner himself or by someone else who accepts the suffering for them.
Jesus was born of a mortal mother and an immortal Father. Because of that unique birthright, Jesus was an infinite Being who was able to live without commiting sin. Therefore He was the only one capable of atoning for our sins in an infinite way, that would pay for an infinite number of sins for an infinite number of people. Christ was foreordained in the pre-earth life to come to earth and suffer for our sins so that justice may be satisfied (1 Peter 1:20).
Actually, it was not His death that paid for our sins but it was the suffering He experienced in the Garden of Gethsemane and on the cross that did it. Giving up His life was necessary so that He could bring about the resurrection of everyone.