The Prophets Speak on Searching Out Our Dead -- Joseph F. Smith

President Joseph F. Smith became Prophet, Seer and Revelator of The Church of Jesus Christ of Latter-day Saints in 1901, thus ushering in the 20th Century for the Church. He was President of the Church for the site dedication of the Cardston Alberta Temple on 27 July 1913, the first outside of the United States, but died before its dedication on 26 - 29 August, 1923 by President Heber J. Grant. He also dedicated the site for the Laie Hawaii Temple 1 June 1915 but died before its dedication on 27 - 30 November 1919 also by President Heber J. Grant. The Laie Hawaii Temple was the 5th Temple built and the first outside of the Continental area of the United States. The Cardston Alberta Temple was the sixth temple built. 

President Joseph F. Smith received the important revelation recorded in the Doctrine and Covenants as Section 138. Speaking of the mission of our Savior Jesus Christ, President Smith wrote:

29 And as I wondered, my eyes were opened, and my understanding quickened, and I perceived that the Lord went not in person among the wicked and the disobedient who had rejected the truth, to teach them;

30 But behold, from among the righteous, he organized his forces and appointed messengers, clothed with power and authority, and commissioned them to go forth and carry the light of the gospel to them that were in darkness, even to fall the spirits of men; and thus was the gospel preached to the dead.

31 And the chosen messengers went forth to declare the acceptable day of the Lord and proclaim liberty to the captives who were bound, even unto all who would repent of their sins and receive the gospel.

32 Thus was the gospel preached to those who had died in their sins, without a knowledge of the truth, or in transgression, having rejected the prophets.

33 These were taught faith in God, repentance from sin, vicarious baptism for the cremission of sins, the gift of the Holy Ghost by the laying on of hands,

34 And all other principles of the gospel that were necessary for them to know in order to qualify themselves that they might be judged according to men in the flesh, but live according to God in the spirit.

 President Smith left many other great and marvelous teachings about the importance of the redemption of the dead and the vicarious work for the dead in the Temples. The following quotes are taken from

Smith, Joseph Fielding. Gospel Doctrine: Selections from the Sermons and Writings of Joseph F. Smith. Salt Lake City, Utah: Deseret Book Company, 1959.

This is the work that we have to perform. God has shown us the way and given us the means by which we may consummate and fill our mission upon this earth and perfect our destiny; for we are destined and foreordained to become like God, and unless we do become like him we will never be permitted to dwell with him. When we become like him you will find that we will be presented before him in the form in which we were created, male and female. The woman will not go there alone, and the man will not go there alone, and claim exaltation. They may attain a degree of salvation alone, but when they are exalted they will be exalted according to the law of the celestial kingdom. They cannot be exalted in any other way, neither the living nor the dead. It is well for us to learn something about why we build temples, and why we administer in them for the dead as well as for the living. We do this that we may become like unto him, and dwell with him eternally; that we may become sons of God, heirs of God and joint heirs with Jesus Christ. Tabernacle Sermon, June 12, 1898.

President Smith explained that we must do the work for all of those who have departed this life and that the efficacy of the ordinances depends on the acceptance of those living in the Spirit World. If they do not accept the Gospel, they will have not benefit from the ordinances performed. 

In relation to the deliverance of spirits from their prison house, of course, we believe that can only be done after the gospel has been preached to them in the spirit, and they have accepted the same, and the work necessary to their redemption by the living be done for them. That this work may be hastened so that all who believe, in the spirit world, may receive the benefit of deliverance, it is revealed that the great work of the Millennium shall be the work in the temples for the redemption of the dead; and then we hope to enjoy the benefits of revelation through the Urim and Thummim, or by such means as the Lord may reveal concerning those for whom the work shall be done, so that we may not work by chance, or by faith alone, without knowledge, but with the actual knowledge revealed unto us. It stands to reason that, while the gospel may be preached unto all, the good and the bad, or rather to those who would repent and to those who would not repent in the spirit world, the same as it is here, redemption will only come to those who repent and obey. There is, no doubt, great leniency given to people who are anxious to do the work for their dead, and in some instances, very unworthy people may have the work done for them; it does not follow, however, that they will receive any benefit therefrom, and the correct thing is to do the work only for those of whom we have the testimony that they will receive it. However, we are disposed to give the benefit of the doubt to the dead, as it is better to do the work for many who are unworthy than to neglect one who is worthy. Now, we know in part and see in part, but steadfastly look forward to the time when that which is perfect will come. We are left largely to our own agency here, to exercise our own intelligence and to receive all the light that is revealed so far as we are capable of receiving it, and only those who seek the light, and desire it, are likely to find it. Improvement Era, Vol. 5, December, 1901, pp. 145-147.

About the importance of the work for the salvation of the dead, President Smith wrote:

Administering ordinances that God has revealed are essential to the salvation of the living and their preparation for greater exaltation and glory here and hereafter, and also for the redemption of the dead. What can you think of greater than this? To my mind, there isn't anything so great and so glorious in this world as to labor for the salvation of the living and for the redemption of the dead. We read of the Savior going to preach the gospel to the spirits in prison, when his body lay in the tomb. That was a part of the great mission he had to perform. He was sent not only to preach the gospel to those dwelling in mortality, but he was foreordained and anointed of God to open the doors of the prison house to those in bondage and to proclaim his gospel to them. Young Woman's Journal,Vol. 23, 1911, pp. 128-132.