In our concept of the after life, the spirit world is divided into to regions. Paradise is the place where the righteous spirits go. This is where those that did their best to keep the commandments, who repented when they fell short, who had faith in Christ, and who received the saving ordinances wait for the resurrection. The other part of the spirit world is called spirit prison. This is where those spirits go who, for whatever reason, did not meet the requirements for entry in to paradise. This is the place where these spirits pay for their sins. This is hell. The spirits of the righteous can and do work among those in prison teaching them the gospel of Jesus Christ. Those that did not have an adequate opportunity to accept the gospel in this life then have the opportunity to accept it in the spirit world. If they do accept it their, then they may be released from spirit prison by also accepting the saving ordinances. However, these ordinances can only be performed by mortals. So, we perform then on the behalf of the dead in our temples. Finally, we have to ask how do we know who has had and adequate opportunity to accept the gospel and who hasn't. The answer is that we don't. That judgment is not ours to make. It is God's. So, we do the ordinances for everyone and leave it up to God to decide who is worthy.