14 Howbeit that was not first which is spiritual, but that which is natural; and afterward that which is spiritual. The first man is of the earth, earthy; the second man is the Lord from heaven. As is the earthy, such are they also that are earthy; and as is the heavenly, such are they also that are heavenly. And as are the records on the earth in relation to your dead, which are truly made out, so also are the records in heaven. This, therefore, is the sealing and binding power, and, in one sense of the word, the keys of the kingdom, which consist in the key of knowledge
15 And now, my dearly beloved brethren and sisters, let me assure you that these are principles in relation to the dead and the living that cannot be lightly passed over, as pertaining to our salvation. For their salvation is necessary and essential to our salvation, as Paul says concerning the fathers—that they without us cannot be made perfect—neither can we without our dead be made perfect.
In Section 128, the Prophet expounds on Malachi 4:5-6 and explains that baptism for the dead is "a welding link" between parents and children (D&C 128:18). Verse 15 makes it clear that fathers and children cannot be saved without each other, and that this ordinance of baptism for the dead was instigated to allow children to save their fathers, and perhaps for fathers to also save their dead children. In verse 14 he further explains that unless children are sealed by temple ordinances to their deceased forebears, who are in turn sealed to each other in God's family, neither can be fully saved and exalted. The themes outlined in this section clearly relate baptism for the dead to other patriarchal priesthood practices outlined in Moses 6:1-5.