”Come all of you … and with all your precious things of the earth, and build a house unto my name for the Most High to dwell therein. For there is not place found on the earth that he may come and restore again that which was lost unto you, or which he has taken away, even the fullness of the Priesthood.” (LDS D&C 124:25-28, RE T&C 141:10)
When you have part of something, you are still missing stuff until you obtain a fulness. When you have the fulness of that thing, what you have is complete, whole or in other words perfect. At that point, there is no need to continue pursuing that thing because what you already have is complete. If you keep messing with it by adding to or taking away from what you have, what you are doing is either an admission that what you have is not really the fulness and you’re continuing to work until you do obtain the fulness or what you are doing is apostatizing from what you have been given.
There are various ways that this idea of fulness applies. I’ll mention a few: prophecy, the gospel and the priesthood.
When it comes to prophecy, there can be, and is often, a partial fulfillment of prophecy which happens many times. Something happens that appears to be directly aligned with a prophecy and then something else happens that also appears to be directly aligned with that prophecy and you end up getting a lot of ways a prophecy appears to have come true. Each one of those can be true enough in itself. It isn’t to take away any validity from each or every instance. But in the grand scheme, the prophecy is not truly fulfilled until it has been fulfilled every whit, from beginning to end in a complete and whole way. When that happens, then, and only then is the prophecy truly fulfilled. At that point you have experienced a fulness of that prophecy.
Jesus is a good example of this with many biblical prophecies. There were often other ways for a prophecy to be fulfilled and some continue to happen in some form or other up to this day. But the whole and complete prophecy, which fulfills every jot and tittle is not done except in the life, teachings and mission of Jesus throughout his life, including his resurrection and his continued ministry until his final return and the winding up scenes of this existence. When it is all complete and the work is done, then will all prophecy be fulfilled. That is the fulness of prophecy.
The gospel of Jesus Christ is and has been taught in various forms throughout the ages of the earth. Sometimes parts or portions of the gospel are taught among people who have no reference to Jesus at all. Yet they do have some of the gospel. Each piece is indeed true enough on its own to merit some credibility and deserves credence to it. In order to have a fulness of the gospel, it is not enough to just have some pieces. You need whatever it takes to make it whole or complete or perfect. When you have the fulness of the gospel, you have everything you need to experience all that the gospel offers. You lack nothing. It is all there for you. You just have to live it to experience it.
The book of Mormon contains the fulness of the gospel of Jesus Christ. What is contained within that record is sufficient to lead a man or woman to experience all that the gospel offers, which culminates in being brought back into the presence of God, having been cleansed from all sin and being redeemed from the fall. The teachings contained within the book of Mormon are sufficient to accomplish all of that. Could there be more witnesses to the fulness of the gospel? of course there can be. Additional witnesses do not add to the fulness of gospel of Jesus Christ, nor do they take anything away from it. More witnesses serve to clarify, reinforce and build confidence in the truth of what is already there in the fulness of the gospel.
The fulness of the priesthood is something that is entirely different from the fulness of the gospel. They are certainly related but they are also quite different. Individuals have experienced the fulness of the gospel throughout the ages of the world. The scriptures demonstrate how mankind at various times and in various places have been redeemed, being brought back into the presence of God. When this happens for an individual, that individual has experienced the fulness of the gospel of Jesus Christ. But that does not mean that it fulfills all prophecy. One person experiencing the fulness of the gospel is a piece of fulfilling prophecy but a lot more needs to happen in order for all things to be fulfilled. What needs to happen ultimately is a complete reunion and reconciliation of heaven and earth, so that, as Jesus prays, the will of God is done on earth as it is in heaven. There is no difference. At that point, heaven and earth would be reunited and be “one” again, as it was in the beginning when Adam and Even walked the earth and also were in the presence of God. Accomplishing all of that certainly includes having the fulness of the gospel of Jesus Christ. That is absolutely critical, indispensable, you can’t do without it. It must be had in its fulness in order for this complete fulfillment to even be a possibility.
To restore the complete connection between heaven and earth is where the fulness of the priesthood comes in. It is the fulness of the priesthood that the initial scripture above is talking about. After having restore the fulness of the gospel of Jesus Christ, in order to fulfill all prophecy concerning the fulness of time, it is necessary for there also to be the fulness of the priesthood. In the verse above, we are informed that this fulness was lost, or not complete. Even though there had been experience with receiving priesthood of Aaron that was never to be taken again from the earth until the sons of Levi do offer again an offering unto the Lord in righteousness — that restoration did not answer to having the fulness of the priesthood. Likewise when there was higher priesthood revealed which had the power of laying on hands for the gift of the holy ghost, this too did not answer to obtaining a fulness of the priesthood. Those may be parts or portions of it, but the fulness of the priesthood remained unfulfilled. This fulness is what was to be restored in the house of the Lord.
If you obtain a fulness of something, you have all that you need. What you have is complete, whole, or perfect. If you have the fulness of the priesthood, there is no need to add anything else to obtain it. What you have is complete. If you take away from what you have, then you have lost the fulness. So if you claim that an ongoing restoration is needed when it comes to what was to be restored in the house of the Lord, then you are either admitting that you have not obtained the fulness of the priesthood that was the intent for the house in the first place. Or what you are doing is actually taking away from the fulness that was once obtained there. Either way, changing or keeping up with an on-going restoration in the temple, indicates that whatever is or has been going on there is not complete, not whole, not perfect and is not a fulness of the priesthood. The original purpose has still not been accomplished.