D&C 128

As the Prophet Joseph Smith introduced what may have seemed a brand new doctrine regarding the salvation of the dead, he wrote -

"You may think this order of things to be very particular; but let me tell you that it is only to answer the will of God, by conforming to the ordinance and preparation that the Lord ordained and prepared before the foundation of the world, for the salvation of the dead who should die without a knowledge of the gospel.' (Doctrine and Covenants Section 128:5)

In his explanation, the emphasis seemed to be not only on the ordinances for the dead but on the authority to administer in the ordinances for the dead. The prophet quoted the words of the Savior anciently -

"...I say also unto thee, That thou art Peter, and upon this rock I will build my church; and the gates of hell shall not prevail against it. And I will give unto thee the keys of the kingdom of heaven: and whatsoever thou shalt bind on earth shall be bound in heaven; and whatsoever thou shalt loose on earth shall be loosed in heaven." (Doctrine and Covenants Section 128:10)

It was those keys that provided authority and power to bind in heaven ordinances for the living and the dead. The Prophet explained -

"Now the great and grand secret of the whole matter, and the summum bonum of the whole subject that is lying before us, consists in obtaining the powers of the Holy Priesthood. For him to whom these keys are given there is no difficulty in obtaining a knowledge of facts in relation to the salvation of the children of men, both as well for the dead as for the living." (Doctrine and Covenants Section 128:11)

Where "summum bonum" means -

"the supreme good from which all others are derived" (Merriam-Webster Dictionary)

It was in the restoration of priesthood keys in this dispensation such that -

"...there is no difficulty in obtaining a knowledge of facts in relation to the salvation of the children of men, both as well for the dead as for the living." (Doctrine and Covenants Section 128:11)