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….
18 I might have rendered a plainer translation to this, but it is sufficiently plain to suit my purpose as it stands. It is sufficient to know, in this case, that the earth will be smitten with a curse unless there is a welding link of some kind or other between the fathers and the children, upon some subject or other—and behold what is that subject? It is the baptism for the dead. For we without them cannot be made perfect; neither can they without us be made perfect. Neither can they nor we be made perfect without those who have died in the gospel also; for it is necessary in the ushering in of the dispensation of the fulness of times, which dispensation is now beginning to usher in, that a whole and complete and perfect union, and welding together of dispensations, and keys, and powers, and glories should take place, and be revealed from the days of Adam even to the present time. And not only this, but those things which never have been revealed from the foundation of the world, but have been kept hid from the wise and prudent, shall be revealed unto babes and sucklings in this, the dispensation of the fulness of times. (D&C 128:15,18)
I have pondered that statement about how neither our dead nor us can be made perfect without each other and the vicarious ordinances of the gospel sealing us all together. It has puzzled me a long time, as I have tried to understand it.
Several answers we are given here—that the dispensations have to be welded together so that all have the keys, powers, and glories, and that things must be revealed that have been kept hidden.
After a long thought, I think it all comes down to the importance of developing charity. We can’t be made perfect without gaining every possible measure of charity, and the efforts of family history help us develop charity in fuller measure.
Consider. The world would say that once a person is dead, there is nothing more that can be done for them. But the sealing power of the priesthood helps us to reach beyond the grave into the eternities to help those who the world believes are beyond all help. And it’s no ordinary help; it’s everlasting salvation we can offer. And we can offer this to people that we’ve never even seen in the flesh.
It is one brand of hero who tries to save those he knows. It is another level of heroism to save those one doesn’t know too. And again, its yet another higher level of heroism to try to save in addition those who one has never personally met and never will meet in this life. It's the same kind of charity Christ demonstrated in bearing the burden of sin not just for those He knew, but those He'd never met in His mortal ministry (including us).
So far that helps answer why we can’t be made perfect without them.
But why can’t they be made perfect without us?
Easy answer: They need saving ordinances to access the sanctifying power of the atonement.
Going deeper: It may also have something to do with their new perspective beyond the veil once they have been taught the gospel. They are in a position to see what eternally matters and what doesn’t, and they have to watch us scraping along, making our choices, both good and bad, without the ability to interfere to influence us (except according to the Lord's will). They depend upon us.
How we must tax their patience as they wait for us to do their saving work for them! It might be something like a parent forced to depend on a 5-year-old to drive a car on a mountainous road. It’s easy enough for anyone to turn away in disgust and frustration when forced to watch a loved one make foolish decisions. Far too easy to give up and repudiate. What charity it must require of them in their helplessness to love us even in our short-sightedness! It is something like the love God has for His children.
And I suppose that sooner or later, all of us, even those of us who have already completed our own work, will eventually be in that place of having to silently watch those who come after, loving in spite of what the following generations do.
So, both the giving and receiving of the gift of vicarious ordinances helps develop the charity we need to become perfect.