JOEL - When we sincerely repent we are forgiven for our sins, but justice still requires that someone must take the punishment for the sin we committed. Jesus took on that punishment Himself for us when He suffered and died on the cross. The purpose for forgiveness is so we won't have to suffer for our own sins. Jesus died to make this possible if we sincerely repent.
One of the most controversial issues in Christian theology is whether salvation is the free gift of unmerited grace or is earned through good works. Paul's statement that "a man is justified by faith without the deeds of the law" (Rom. 3:28) is frequently cited to support the former view, while James's statement that "faith without works is dead" (James 2:20) is often quoted in favor of the latter view. The LDS believe that salvation requires both grace and works.
No one is "good enough" to obtain salvation on his own. The important thing is that we make the effort and do our best to live the commandments and serve God and man. When we do this the difference will be made up by the grace of Jesus Christ.
The death and resurrection of Christ would not be pointless. They are essential ingredients that are needed for our salvation. Jesus suffered and died to pay for our sins. He was resurrected so that we could also be resurrected(1 Cor. 15:22) and make it possible to be saved into heaven. But exactly where we end up in heaven after we are resurrected is dependent on what we do here on earth(Matt. 7:21).
According to LDS beliefs, just about everyone will end up in some kingdom of heaven(1 Cor. 15:40-42, John 14:2). Which level of heaven we end up in is dependent on how faithfully we follow the teachings of Jesus while we live our lives here on earth. We don't have to do any good works if we don't want to be exalted into the highest level of Heaven. But this is where God would like us to be so we can live with Him eternally.
The atonement of Jesus Christ is effective for everyone who lived before and after He was born. Those who lived before the time of Christ are now residing as spirits in the spirit world. There they are given the opportunity to hear and accept the gospel of Jesus Christ(1 Peter 3:18-19). If they repent, they take advantage of the suffering He did for their sins so they will not have to suffer. If they accept His gospel they then have the same oportunity to be saved into the highest kingdom following the resurrection. If they do not accept it they will still enter into heaven, but to a lower level where they will be happy, but will not be in the presence of God. Much of this doctrine has been made more clear to us through latter-day revelation.