D&C 110

After reading the four visions or visits of the Savior, Moses, Elias, and Elijah, these were the thoughts that came to my mind.  First, Elder Bruce R. McConkie wrote concerning the "new and everlasting covenant" -

"...When men accept the gospel, they thereby agree or covenant to keep the commandments of God, and he promises or covenants to give them salvation in his kingdom.

 

The gospel is the everlasting covenant because it is ordained by Him who is Everlasting and also because it is everlastingly the same. In all past ages salvation was gained by adherence to its terms and conditions, and that same compliance will bring the same reward in all future ages. Each time this everlasting covenant is revealed it is new to those of that dispensation. Hence the gospel is the new and everlasting covenant.

 

All covenants between God and man are part of the new and everlasting covenant..."  (Mormon Doctrine)

As Moses appeared in the Kirtland Temple, he -

"...committed unto us the keys of the gathering of Israel from the four parts of the earth, and the leading of the ten tribes from the land of the north."  (Doctrine and Covenants | Section 110:11)

In my mind, gathering Israel is missionary work, accepting the gospel of Jesus Christ and the Doctrine of Christ which is the key to salvation.  That is taking on oneself the new and everlasting covenant.  Then -

"...Elias appeared, and committed the dispensation of the gospel of Abraham, saying that in us and our seed all generations after us should be blessed."  (Doctrine and Covenants | Section 110:12)

...which I commented on yesterday which I suggested that this included the keys of the "new and everlasting covenant of marriage."  Then -

"...Elijah the prophet, who was taken to heaven without tasting death, stood before us, and said:
14 Behold, the time has fully come, which was spoken of by the mouth of Malachi—testifying that he [Elijah] should be sent, before the great and dreadful day of the Lord come—
15 To turn the hearts of the fathers to the children, and the children to the fathers, lest the whole earth be smitten with a curse—"  (Doctrine and Covenants | Section 110:13 - 15)

In my mind, Elijah brought the keys related to temple work for the dead, but more important the "sealing keys" where we learn of the necessary importance of this authority and power  -

"...as pertaining to the new and everlasting covenant, it was instituted for the fulness of my glory; and he that receiveth a fulness thereof must and shall abide the law, or he shall be damned, saith the Lord God.
7 And verily I say unto you, that the conditions of this law are these: All covenants, contracts, bonds, obligations, oaths, vows, performances, connections, associations, or expectations, that are not made and entered into and sealed by the Holy Spirit of promise, of him who is anointed, both as well for time and for all eternity, and that too most holy, by revelation and commandment through the medium of mine anointed, whom I have appointed on the earth to hold this power (and I have appointed unto my servant Joseph to hold this power in the last days, and there is never but one on the earth at a time on whom this power and the keys of this priesthood are conferred), are of no efficacy, virtue, or force in and after the resurrection from the dead; for all contracts that are not made unto this end have an end when men are dead."  (Doctrine and Covenants | Section 132:6 - 7)

Thus this sealing power not only binds the family or house of Israel together for ever but seals all individual covenants that comprise the new and everlasting covenant.  Thus after these three visits with keys of authority and power, Elijah summarizes or confirms the necessity of their visit and why this temple was constructed -

"Therefore, the keys of this dispensation are committed into your hands..."  (Doctrine and Covenants | Section 110:16)