D&C 22

As there were some that desired membership in the newly restored Church having been baptized in some manner in another religious organization wanted to join without rebaptism, the Lord instructed the Prophet Joseph Smith -

"BEHOLD, I say unto you that all old covenants have I caused to be done away in this thing; and this is a new and an everlasting covenant, even that which was from the beginning."  (Doctrine and Covenants Section 22:1)

"This thing" in my mind is in reference to the restoration of His Church and His priesthood power and authority.  One may wonder how this new covenant in reference to old covenants could be new if it was from the beginning.  My thoughts are that first, this new covenant or "last covenant' is in reference to previous covenants or "old" covenants which covenants were either made without divine priesthood authority or power or covenants that are of no efficacy, validity, force, or in other words, "dead" or of no avail.  For the Lord explained -

"Wherefore, although a man should be baptized an hundred times it availeth him nothing, for you cannot enter in at the strait gate by the law of Moses, neither by your dead works.
3 For it is because of your dead works that I have caused this last covenant and this church to be built up unto me, even as in days of old."  (Doctrine and Covenants  Section 22:2 - 3)

Thus the "...and an everlasting covenant" is not dead but eternal being authorized and administered by His priesthood.  Such everlasting covenant by his authorization has been around even from the days of Adam.  Covenants made that are not His covenants by His authority or covenants that are not everlasting are "dead" covenants.