Here Jacob wants to remind them of what he had seen in his vision. He saw Jesus ministering to the Jews in Jerusalem. He saw that the Savior would be persecuted and die in order to open the gates of the resurrection for all mankind. He will heal the consequences of the fall which separated us from the Father. Now in the sixth verse he says that death is part of the merciful plan of God. This is a strange statement. What is merciful about death?
To better understand the thoughts of Jacob, we need to know why death is a meaningful part of God's plan. Modern revelation has explained that our the temporal bodies cannot be exalted. They can be quickened, transfigured and even translated like the City of Enoch, and when that happens the seeds of death are temporarily suspended. But temporal bodies cannot be eternally exalted or return to the presence of God to live with him in eternal glory unless they have gone through the process of death. This can only be achieved by separating the spirit from the physical body so that the spirit can be quickened and then used as a catalyst to quicken the physical temporal body. We may not understand all of the details, but it is just part of God science. Now the separation of the spirit from the body doesn't have to be a long time. It can be in the twinkling of an eye, but the separation must take place.
Part of this doctrine is found in Alma 42:8-9 and the rest of it will be found in the Doctrine and Covenants 88:28.There the Lord says he must exalt the spirit first because it is the glory of the spirit that determines the exalted glory of the physical body in its resurrected state. And because this is the only way it can be done, Jacob is correct is saying that death is indeed part of the merciful plan of God.