"One great objective of our lives should be to make our calling and election sure, that is, to so live that we receive assurance from the Lord that when this life is over, we shall be exalted and dwell with him. Mormon may have had this blessing, as did other Nephite prophets (see Mosiah 26:20; Enos 1:27; 3 Nephi 28:3), for he tells us: ’I know that I shall be lifted up at the last day.’ (Mormon 2:19)
’Those members of the Church who devote themselves wholly to righteousness, living by every word that proceedeth forth from the mouth of God, make their calling and election sure. That is, they receive the more sure word of prophecy, which means that the Lord seals their exaltation upon them while they are yet in this life…
’…The more sure word of prophecy means a man’s knowing that he is sealed up unto eternal life, by revelation and the spirit of prophecy through the power of the Holy Priesthood.’ (D. & C. 131:5.)…The prophet, for one, had this seal placed upon him…To him Deity said: ’I am the Lord thy God, and will be with thee even unto the end of the world, and through all eternity; for verily I seal upon you your exaltation, and prepare a throne for you in the kingdom of my Father with Abraham your father.’ (D. & C. 132:49.)’ (Bruce R. McConkie, Mormon Doctrine, pp. 109-110)"
(Book of Mormon Student Manual, 1981, p. 459-60)