Based on the eternal principles of vicarious service, the Lord has ordained baptism for the dead as the means whereby all his worthy children of all ages can become heirs of salvation in his kingdom.(ibid. p. 73)
Sometime after birth into this life and before resurrection and judgement, every living soul will hear the gospel message and be judged by his reaction thereto. Millions who pass to the spirit world without receiving an opportunity during mortality to hear the truths of salvation will receive their chance subsequent to what men call death.
The great principles and procedures whereby the saving truths of the gospel are offered to, accepted by, and made binding upon the departed dead, comprise the doctrine of salvation for the dead. Pursuant to this doctrine the principles of salvation are taught in the spirit world, leaving the ordinances thereof to be performed in this life on a vicarious - proxy basis. By accepting the gospel in the spirit world, and because the ordinances of salvation and exaltation are performed vicariously in this world, the worthy dead can become heirs of the fulness of the Father's kingdom...
For the Latter-day Saints (LDS), salvation for the dead is the system whereunder those who would have accepted the gospel in this life had they been permitted to hear it, will have the chance to accept it in the spirit world, and will them be entitled to all the blessings which passed them by in mortality.(ibid. pp. 672-673).