The 144,000 are the most important group of people that have ever lived. The second coming doesn't happen until this group is formed and sealed according to Revelation chapter 7. Why are they so important? The 144,000 live on earth without a mediator and do not sin during this period of time. Rev. 22:11. But that is not the only reason that they are important. They prove that all of the other people who will be in heaven (called the great multitude in Rev. 7:9) are safe to save and will not sin in heaven even though they had not yet completely overcome sin in this life. They were overcoming sin but still had victories to win when their lives ended. God says with the 144,000 if they can completely overcome sin being the weakest generation that has ever lived then the redeemed in heaven no longer possessing a fallen nature will never sin again. The 144,000 are the last generation and the farthest from creation. Their natures have degraded the most of any other people who have lived before. They are baptized in the terrible ordeal they go through for all of humanity that is saved. In The Great Controversy page 630 and 631 we are told the 144 must, "be baptized with the baptism." This is probably what 1 Corinthians 15:29 is referring to when it talks about baptism for the dead. I welcome anyone who would like to discuss this further.