This is an excerpt from A Beginner's Guide to Talmage by Calvin R. Stephens.
Plan of Salvation Includes Those in the Spirit World. We learn that the ministry of Christ was not confined to the few who lived in mortality during the short period of His earthly life, nor to them and the generations then future; but to all, dead, living, and yet unborn. It cannot be denied that myriads had lived and died before the meridian of time, and of these multitudes, as of the many since born, unnumbered hosts have died without a knowledge of the Gospel and its prescribed plan of salvation. What is their condition, as indeed what shall be the state of the present inhabitants of earth, and of the multitudes yet future, who shall die in ignorance and without the faith that saves? Let us ask again, how can those who know not Christ have faith in Him, and how, while lacking both knowledge and faith can they avail themselves of the provision made for their salvation? The Church of Jesus Christ of Latter-day Saints affirms that the plan of salvation is not bounded by the grave; but that the Gospel is deathless and everlasting, reaching back into the ages that have gone, and forward into the eternities of the future. The ministry of the Savior among the dead doubtless included the revelation of His own atoning death, the inculcation of faith in Himself and in the divinely-appointed plan He represented, and the necessity of a repentance acceptable unto God. It is reasonable to believe that the other essential requirements comprised within the laws and ordinances of the Gospel were made known.
A Beginner’s Guide to Talmage, Calvin R. Stephens, pp. 32-3.