"The fundamental principles of our religion are the testimony of the Apostles and Prophets, concerning Jesus Christ, that He died, was buried, and rose again the third day, and ascended into heaven; and all other things which pertain to our religion are only appendages to it. But in connection with these, we believe in the gift of the Holy Ghost, the power of faith, the enjoyment of the spiritual gifts according to the will of God, the restoration of the house of Israel, and the final triumph of truth."
-- Teachings of Presidents of the Church: Joseph Smith, (2011), 45–56.
"Jesus is the first begotten from the dead, as you will understand. Neither Enoch, Elijah, Moses, nor any other man that ever lived on earth, no matter how strictly he lived, ever obtained a resurrection until after Jesus Christ’s body was called from the tomb by the angel. He was the first begotten from the dead. He is the Master of the resurrection—the first flesh that lived here after receiving the glory of the resurrection."
-- Teachings of Presidents of the Church: Brigham Young, (1997), 272–78.
"When Jesus came, He came to do a work in many particulars similar to that in which we are engaged, and when He got through with His work here, He stood as the Savior of the world, and of the human family. He came to preach the Gospel to the poor, to open the prison doors to those that were imprisoned, to set them at liberty, and to proclaim the acceptable hour of the Lord, etc. This was a work connected with the people who lived at the time of the flood and were destroyed and kept in prison until the Lord should see proper to extend manifestations of His mercy to them. Hence, as we read, “Christ hath once suffered for sins, the just for the unjust, that He might bring us to God, being put to death in the flesh, but quickened by the spirit; by which also he went and preached to the spirits in prison: which sometime were disobedient when once the long suffering of God waited in the days of Noah.” [See 1 Peter 3:18–20.] He having finished His work upon earth for the living, went and performed a work for the dead; as we are informed, “He went and preached to spirits in prison that had sometime been disobedient in the days of Noah.
It is reserved for us to do a work for those who have passed away who have not obeyed or had the Gospel in their lifetime. We are here to do a work connected with the redemption of the dead. When the Temple was commanded to be built in Nauvoo, after the Temple had been built in Kirtland, and after so many keys had been turned, and after so many manifestations, visions and ministrations had been had, yet it was said then that there was not a place upon the earth in which to perform the ordinance of baptism for the dead, and Joseph was commanded to build a house for that purpose."
-- Teachings of Presidents of the Church: John Taylor, (2011), 182–90.